diff --git a/openmc/__init__.py b/openmc/__init__.py
index 026ccce114..a0492ee408 100644
--- a/openmc/__init__.py
+++ b/openmc/__init__.py
@@ -13,10 +13,10 @@ from openmc.settings import *
 from openmc.surface import *
 from openmc.universe import *
 from openmc.mesh import *
-from openmc.mgxs_library import *
 from openmc.filter import *
 from openmc.trigger import *
 from openmc.tallies import *
+from openmc.mgxs_library import *
 from openmc.cmfd import *
 from openmc.executor import *
 from openmc.statepoint import *
diff --git a/openmc/mgxs/library.py b/openmc/mgxs/library.py
index ee11d0ef64..5685681e43 100644
--- a/openmc/mgxs/library.py
+++ b/openmc/mgxs/library.py
@@ -11,6 +11,7 @@ import numpy as np
 import openmc
 import openmc.mgxs
 import openmc.checkvalue as cv
+from openmc.tallies import ESTIMATOR_TYPES
 
 
 if sys.version_info[0] >= 3:
@@ -39,7 +40,7 @@ class Library(object):
     mgxs_types : Iterable of str
         The types of cross sections in the library (e.g., ['total', 'scatter'])
     name : str, optional
-        Name of the multi-group cross section. library Used as a label to
+        Name of the multi-group cross section library. Used as a label to
         identify tallies in OpenMC 'tallies.xml' file.
 
     Attributes
@@ -64,6 +65,9 @@ class Library(object):
         The highest legendre moment in the scattering matrices (default is 0)
     energy_groups : openmc.mgxs.EnergyGroups
         Energy group structure for energy condensation
+    estimator : str or None
+        The tally estimator used to compute multi-group cross sections. If None,
+        the default for each MGXS type is used.
     tally_trigger : openmc.Trigger
         An (optional) tally precision trigger given to each tally used to
         compute the cross section
@@ -102,6 +106,7 @@ class Library(object):
         self._sp_filename = None
         self._keff = None
         self._sparse = False
+        self._estimator = None
 
         self.name = name
         self.openmc_geometry = openmc_geometry
@@ -206,6 +211,10 @@ class Library(object):
     def tally_trigger(self):
         return self._tally_trigger
 
+    @property
+    def estimator(self):
+        return self._estimator
+
     @property
     def num_groups(self):
         return self.energy_groups.num_groups
@@ -327,6 +336,11 @@ class Library(object):
         cv.check_type('tally trigger', tally_trigger, openmc.Trigger)
         self._tally_trigger = tally_trigger
 
+    @estimator.setter
+    def estimator(self, estimator):
+        cv.check_value('estimator', estimator, ESTIMATOR_TYPES)
+        self._estimator = estimator
+
     @sparse.setter
     def sparse(self, sparse):
         """Convert tally data from NumPy arrays to SciPy list of lists (LIL)
@@ -368,9 +382,11 @@ class Library(object):
                 mgxs.domain_type = self.domain_type
                 mgxs.energy_groups = self.energy_groups
                 mgxs.by_nuclide = self.by_nuclide
+                if self.estimator is not None:
+                    mgxs.estimator = self.estimator
 
                 # If a tally trigger was specified, add it to the MGXS
-                if self.tally_trigger:
+                if self.tally_trigger is not None:
                     mgxs.tally_trigger = self.tally_trigger
 
                 # Specify whether to use a transport ('P0') correction
diff --git a/openmc/mgxs/mgxs.py b/openmc/mgxs/mgxs.py
index 7aa12d0418..d68dc508f2 100644
--- a/openmc/mgxs/mgxs.py
+++ b/openmc/mgxs/mgxs.py
@@ -13,6 +13,7 @@ import numpy as np
 
 import openmc
 import openmc.checkvalue as cv
+from openmc.tallies import ESTIMATOR_TYPES
 from openmc.mgxs import EnergyGroups
 
 if sys.version_info[0] >= 3:
@@ -39,7 +40,6 @@ MGXS_TYPES = ['total',
               'inverse-velocity',
               'prompt-nu-fission']
 
-
 # Supported domain types
 DOMAIN_TYPES = ['cell',
                 'distribcell',
@@ -102,7 +102,7 @@ class MGXS(object):
     tally_keys : list of str
         The keys into the tallies dictionary for each tally used to compute
         the multi-group cross section
-    estimator : {'tracklength', 'analog'}
+    estimator : {'tracklength', 'collision', 'analog'}
         The tally estimator used to compute the multi-group cross section
     tallies : collections.OrderedDict
         OpenMC tallies needed to compute the multi-group cross section
@@ -144,11 +144,11 @@ class MGXS(object):
 
     def __init__(self, domain=None, domain_type=None,
                  energy_groups=None, by_nuclide=False, name=''):
-
         self._name = ''
         self._rxn_type = None
         self._by_nuclide = None
         self._nuclides = None
+        self._estimator = 'tracklength'
         self._domain = None
         self._domain_type = None
         self._energy_groups = None
@@ -160,6 +160,7 @@ class MGXS(object):
         self._loaded_sp = False
         self._derived = False
         self._hdf5_key = None
+        self._valid_estimators = ESTIMATOR_TYPES
 
         self.name = name
         self.by_nuclide = by_nuclide
@@ -250,7 +251,7 @@ class MGXS(object):
 
     @property
     def estimator(self):
-        return 'tracklength'
+        return self._estimator
 
     @property
     def tallies(self):
@@ -368,6 +369,11 @@ class MGXS(object):
         cv.check_iterable_type('nuclides', nuclides, basestring)
         self._nuclides = nuclides
 
+    @estimator.setter
+    def estimator(self, estimator):
+        cv.check_value('estimator', estimator, self._valid_estimators)
+        self._estimator = estimator
+
     @domain.setter
     def domain(self, domain):
         cv.check_type('domain', domain, _DOMAINS)
@@ -1643,7 +1649,7 @@ class MatrixMGXS(MGXS):
     tally_keys : list of str
         The keys into the tallies dictionary for each tally used to compute
         the multi-group cross section
-    estimator : {'tracklength', 'analog'}
+    estimator : {'tracklength', 'collision', 'analog'}
         The tally estimator used to compute the multi-group cross section
     tallies : collections.OrderedDict
         OpenMC tallies needed to compute the multi-group cross section
@@ -1692,10 +1698,6 @@ class MatrixMGXS(MGXS):
 
         return [[energy], [energy, energyout]]
 
-    @property
-    def estimator(self):
-        return 'analog'
-
     def get_xs(self, in_groups='all', out_groups='all',
                subdomains='all', nuclides='all',
                xs_type='macro', order_groups='increasing',
@@ -2072,7 +2074,7 @@ class TotalXS(MGXS):
     tally_keys : list of str
         The keys into the tallies dictionary for each tally used to compute
         the multi-group cross section
-    estimator : {'tracklength', 'analog'}
+    estimator : {'tracklength', 'collision', 'analog'}
         The tally estimator used to compute the multi-group cross section
     tallies : collections.OrderedDict
         OpenMC tallies needed to compute the multi-group cross section. The keys
@@ -2190,7 +2192,7 @@ class TransportXS(MGXS):
     tally_keys : list of str
         The keys into the tallies dictionary for each tally used to compute
         the multi-group cross section
-    estimator : {'tracklength', 'analog'}
+    estimator : 'analog'
         The tally estimator used to compute the multi-group cross section
     tallies : collections.OrderedDict
         OpenMC tallies needed to compute the multi-group cross section. The keys
@@ -2233,6 +2235,8 @@ class TransportXS(MGXS):
         super(TransportXS, self).__init__(domain, domain_type,
                                           groups, by_nuclide, name)
         self._rxn_type = 'transport'
+        self._estimator = 'analog'
+        self._valid_estimators = ['analog']
 
     @property
     def scores(self):
@@ -2245,10 +2249,6 @@ class TransportXS(MGXS):
         energyout_filter = openmc.Filter('energyout', group_edges)
         return [[energy_filter], [energy_filter], [energyout_filter]]
 
-    @property
-    def estimator(self):
-        return 'analog'
-
     @property
     def rxn_rate_tally(self):
         if self._rxn_rate_tally is None:
@@ -2320,7 +2320,7 @@ class NuTransportXS(TransportXS):
     tally_keys : list of str
         The keys into the tallies dictionary for each tally used to compute
         the multi-group cross section
-    estimator : {'tracklength', 'analog'}
+    estimator : 'analog'
         The tally estimator used to compute the multi-group cross section
     tallies : collections.OrderedDict
         OpenMC tallies needed to compute the multi-group cross section. The keys
@@ -2441,7 +2441,7 @@ class AbsorptionXS(MGXS):
     tally_keys : list of str
         The keys into the tallies dictionary for each tally used to compute
         the multi-group cross section
-    estimator : {'tracklength', 'analog'}
+    estimator : {'tracklength', 'collision', 'analog'}
         The tally estimator used to compute the multi-group cross section
     tallies : collections.OrderedDict
         OpenMC tallies needed to compute the multi-group cross section. The keys
@@ -2557,7 +2557,7 @@ class CaptureXS(MGXS):
     tally_keys : list of str
         The keys into the tallies dictionary for each tally used to compute
         the multi-group cross section
-    estimator : {'tracklength', 'analog'}
+    estimator : {'tracklength', 'collision', 'analog'}
         The tally estimator used to compute the multi-group cross section
     tallies : collections.OrderedDict
         OpenMC tallies needed to compute the multi-group cross section. The keys
@@ -2679,7 +2679,7 @@ class FissionXS(MGXS):
     tally_keys : list of str
         The keys into the tallies dictionary for each tally used to compute
         the multi-group cross section
-    estimator : {'tracklength', 'analog'}
+    estimator : {'tracklength', 'collision', 'analog'}
         The tally estimator used to compute the multi-group cross section
     tallies : collections.OrderedDict
         OpenMC tallies needed to compute the multi-group cross section. The keys
@@ -2790,7 +2790,7 @@ class NuFissionXS(MGXS):
     tally_keys : list of str
         The keys into the tallies dictionary for each tally used to compute
         the multi-group cross section
-    estimator : {'tracklength', 'analog'}
+    estimator : {'tracklength', 'collision', 'analog'}
         The tally estimator used to compute the multi-group cross section
     tallies : collections.OrderedDict
         OpenMC tallies needed to compute the multi-group cross section. The keys
@@ -2906,7 +2906,7 @@ class KappaFissionXS(MGXS):
     tally_keys : list of str
         The keys into the tallies dictionary for each tally used to compute
         the multi-group cross section
-    estimator : {'tracklength', 'analog'}
+    estimator : {'tracklength', 'collision', 'analog'}
         The tally estimator used to compute the multi-group cross section
     tallies : collections.OrderedDict
         OpenMC tallies needed to compute the multi-group cross section. The keys
@@ -3019,7 +3019,7 @@ class ScatterXS(MGXS):
     tally_keys : list of str
         The keys into the tallies dictionary for each tally used to compute
         the multi-group cross section
-    estimator : {'tracklength', 'analog'}
+    estimator : {'tracklength', 'collision', 'analog'}
         The tally estimator used to compute the multi-group cross section
     tallies : collections.OrderedDict
         OpenMC tallies needed to compute the multi-group cross section. The keys
@@ -3134,7 +3134,7 @@ class NuScatterXS(MGXS):
     tally_keys : list of str
         The keys into the tallies dictionary for each tally used to compute
         the multi-group cross section
-    estimator : {'tracklength', 'analog'}
+    estimator : {'tracklength', 'collision', 'analog'}
         The tally estimator used to compute the multi-group cross section
     tallies : collections.OrderedDict
         OpenMC tallies needed to compute the multi-group cross section. The keys
@@ -3177,10 +3177,8 @@ class NuScatterXS(MGXS):
         super(NuScatterXS, self).__init__(domain, domain_type,
                                           groups, by_nuclide, name)
         self._rxn_type = 'nu-scatter'
-
-    @property
-    def estimator(self):
-        return 'analog'
+        self._estimator = 'analog'
+        self._valid_estimators = ['analog']
 
 
 class ScatterMatrixXS(MatrixMGXS):
@@ -3268,7 +3266,7 @@ class ScatterMatrixXS(MatrixMGXS):
     tally_keys : list of str
         The keys into the tallies dictionary for each tally used to compute
         the multi-group cross section
-    estimator : {'tracklength', 'analog'}
+    estimator : 'analog'
         The tally estimator used to compute the multi-group cross section
     tallies : collections.OrderedDict
         OpenMC tallies needed to compute the multi-group cross section. The keys
@@ -3314,6 +3312,8 @@ class ScatterMatrixXS(MatrixMGXS):
         self._correction = 'P0'
         self._legendre_order = 0
         self._hdf5_key = 'scatter matrix'
+        self._estimator = 'analog'
+        self._valid_estimators = ['analog']
 
     def __deepcopy__(self, memo):
         clone = super(ScatterMatrixXS, self).__deepcopy__(memo)
@@ -3929,7 +3929,7 @@ class NuScatterMatrixXS(ScatterMatrixXS):
     tally_keys : list of str
         The keys into the tallies dictionary for each tally used to compute
         the multi-group cross section
-    estimator : {'tracklength', 'analog'}
+    estimator : 'analog'
         The tally estimator used to compute the multi-group cross section
     tallies : collections.OrderedDict
         OpenMC tallies needed to compute the multi-group cross section. The keys
@@ -4051,7 +4051,7 @@ class MultiplicityMatrixXS(MatrixMGXS):
     tally_keys : list of str
         The keys into the tallies dictionary for each tally used to compute
         the multi-group cross section
-    estimator : {'tracklength', 'analog'}
+    estimator : 'analog'
         The tally estimator used to compute the multi-group cross section
     tallies : collections.OrderedDict
         OpenMC tallies needed to compute the multi-group cross section. The keys
@@ -4094,6 +4094,8 @@ class MultiplicityMatrixXS(MatrixMGXS):
         super(MultiplicityMatrixXS, self).__init__(domain, domain_type, groups,
                                                    by_nuclide, name)
         self._rxn_type = 'multiplicity matrix'
+        self._estimator = 'analog'
+        self._valid_estimators = ['analog']
 
     @property
     def scores(self):
@@ -4198,7 +4200,7 @@ class NuFissionMatrixXS(MatrixMGXS):
     tally_keys : list of str
         The keys into the tallies dictionary for each tally used to compute
         the multi-group cross section
-    estimator : {'tracklength', 'analog'}
+    estimator : 'analog'
         The tally estimator used to compute the multi-group cross section
     tallies : collections.OrderedDict
         OpenMC tallies needed to compute the multi-group cross section. The keys
@@ -4242,6 +4244,8 @@ class NuFissionMatrixXS(MatrixMGXS):
                                                 groups, by_nuclide, name)
         self._rxn_type = 'nu-fission'
         self._hdf5_key = 'nu-fission matrix'
+        self._estimator = 'analog'
+        self._valid_estimators = ['analog']
 
 
 class Chi(MGXS):
@@ -4313,7 +4317,7 @@ class Chi(MGXS):
     tally_keys : list of str
         The keys into the tallies dictionary for each tally used to compute
         the multi-group cross section
-    estimator : {'tracklength', 'analog'}
+    estimator : 'analog'
         The tally estimator used to compute the multi-group cross section
     tallies : collections.OrderedDict
         OpenMC tallies needed to compute the multi-group cross section. The keys
@@ -4355,6 +4359,8 @@ class Chi(MGXS):
                  groups=None, by_nuclide=False, name=''):
         super(Chi, self).__init__(domain, domain_type, groups, by_nuclide, name)
         self._rxn_type = 'chi'
+        self._estimator = 'analog'
+        self._valid_estimators = ['analog']
 
     @property
     def scores(self):
@@ -4372,10 +4378,6 @@ class Chi(MGXS):
     def tally_keys(self):
         return ['nu-fission-in', 'nu-fission-out']
 
-    @property
-    def estimator(self):
-        return 'analog'
-
     @property
     def rxn_rate_tally(self):
         if self._rxn_rate_tally is None:
@@ -4803,7 +4805,7 @@ class ChiPrompt(Chi):
     tally_keys : list of str
         The keys into the tallies dictionary for each tally used to compute
         the multi-group cross section
-    estimator : {'tracklength', 'analog'}
+    estimator : 'analog'
         The tally estimator used to compute the multi-group cross section
     tallies : collections.OrderedDict
         OpenMC tallies needed to compute the multi-group cross section. The keys
@@ -4918,7 +4920,7 @@ class InverseVelocity(MGXS):
     tally_keys : list of str
         The keys into the tallies dictionary for each tally used to compute
         the multi-group cross section
-    estimator : {'tracklength', 'analog'}
+    estimator : {'tracklength', 'collision', 'analog'}
         The tally estimator used to compute the multi-group cross section
     tallies : collections.OrderedDict
         OpenMC tallies needed to compute the multi-group cross section. The keys
@@ -5052,7 +5054,7 @@ class PromptNuFissionXS(MGXS):
     tally_keys : list of str
         The keys into the tallies dictionary for each tally used to compute
         the multi-group cross section
-    estimator : {'tracklength', 'analog'}
+    estimator : {'tracklength', 'collision', 'analog'}
         The tally estimator used to compute the multi-group cross section
     tallies : collections.OrderedDict
         OpenMC tallies needed to compute the multi-group cross section. The keys
diff --git a/openmc/tallies.py b/openmc/tallies.py
index c68b0faacb..f645fa1624 100644
--- a/openmc/tallies.py
+++ b/openmc/tallies.py
@@ -40,6 +40,9 @@ _SCORE_CLASSES = (basestring, CrossScore, AggregateScore)
 _NUCLIDE_CLASSES = (basestring, Nuclide, CrossNuclide, AggregateNuclide)
 _FILTER_CLASSES = (Filter, CrossFilter, AggregateFilter)
 
+# Valid types of estimators
+ESTIMATOR_TYPES = ['tracklength', 'collision', 'analog']
+
 
 def reset_auto_tally_id():
     """Reset counter for auto-generated tally IDs."""
@@ -387,8 +390,7 @@ class Tally(object):
 
     @estimator.setter
     def estimator(self, estimator):
-        cv.check_value('estimator', estimator,
-                       ['analog', 'tracklength', 'collision'])
+        cv.check_value('estimator', estimator, ESTIMATOR_TYPES)
         self._estimator = estimator
 
     @triggers.setter