diff --git a/openmc/_xml.py b/openmc/_xml.py
index b49d192097..32679fd89e 100644
--- a/openmc/_xml.py
+++ b/openmc/_xml.py
@@ -43,3 +43,21 @@ def get_text(elem, name, default=None):
     else:
         child = elem.find(name)
         return child.text if child is not None else default
+
+
+def reorder_attributes(root):
+    """Sort attributes in XML to preserve pre-Python 3.8 behavior
+
+    Parameters
+    ----------
+    root : xml.etree.ElementTree.Element
+        Root element
+
+    """
+    for el in root.iter():
+        attrib = el.attrib
+        if len(attrib) > 1:
+            # adjust attribute order, e.g. by sorting
+            attribs = sorted(attrib.items())
+            attrib.clear()
+            attrib.update(attribs)
diff --git a/openmc/data/library.py b/openmc/data/library.py
index a66260b929..cd0fe0895b 100644
--- a/openmc/data/library.py
+++ b/openmc/data/library.py
@@ -5,7 +5,7 @@ import pathlib
 import h5py
 
 from openmc.mixin import EqualityMixin
-from openmc._xml import clean_indentation
+from openmc._xml import clean_indentation, reorder_attributes
 
 
 class DataLibrary(EqualityMixin):
@@ -112,6 +112,7 @@ class DataLibrary(EqualityMixin):
         clean_indentation(root)
 
         # Write XML file
+        reorder_attributes(root)  # TODO: Remove when support is Python 3.8+
         tree = ET.ElementTree(root)
         tree.write(str(path), xml_declaration=True, encoding='utf-8',
                    method='xml')
diff --git a/openmc/geometry.py b/openmc/geometry.py
index 450f223b19..3becde9a7a 100644
--- a/openmc/geometry.py
+++ b/openmc/geometry.py
@@ -110,6 +110,7 @@ class Geometry:
             p /= 'geometry.xml'
 
         # Write the XML Tree to the geometry.xml file
+        xml.reorder_attributes(root_element)  # TODO: Remove when support is Python 3.8+
         tree = ET.ElementTree(root_element)
         tree.write(str(p), xml_declaration=True, encoding='utf-8')
 
diff --git a/openmc/material.py b/openmc/material.py
index 92f5132c52..2ab36cf7e8 100644
--- a/openmc/material.py
+++ b/openmc/material.py
@@ -12,7 +12,7 @@ import numpy as np
 import openmc
 import openmc.data
 import openmc.checkvalue as cv
-from ._xml import clean_indentation
+from ._xml import clean_indentation, reorder_attributes
 from .mixin import IDManagerMixin
 
 
@@ -1239,6 +1239,7 @@ class Materials(cv.CheckedList):
                 clean_indentation(element, level=1)
                 element.tail = element.tail.strip(' ')
                 fh.write('  ')
+                reorder_attributes(element)  # TODO: Remove when support is Python 3.8+
                 ET.ElementTree(element).write(fh, encoding='unicode')
 
             # Write the <material> elements.
@@ -1247,6 +1248,7 @@ class Materials(cv.CheckedList):
                 clean_indentation(element, level=1)
                 element.tail = element.tail.strip(' ')
                 fh.write('  ')
+                reorder_attributes(element)  # TODO: Remove when support is Python 3.8+
                 ET.ElementTree(element).write(fh, encoding='unicode')
 
             # Write the closing tag for the root element.
diff --git a/openmc/plots.py b/openmc/plots.py
index 05ce684df9..33020c48a9 100644
--- a/openmc/plots.py
+++ b/openmc/plots.py
@@ -8,7 +8,7 @@ import numpy as np
 
 import openmc
 import openmc.checkvalue as cv
-from ._xml import clean_indentation
+from ._xml import clean_indentation, reorder_attributes
 from .mixin import IDManagerMixin
 
 
@@ -844,5 +844,6 @@ class Plots(cv.CheckedList):
             p /= 'plots.xml'
 
         # Write the XML Tree to the plots.xml file
+        reorder_attributes(self._plots_file)  # TODO: Remove when support is Python 3.8+
         tree = ET.ElementTree(self._plots_file)
         tree.write(str(p), xml_declaration=True, encoding='utf-8')
diff --git a/openmc/settings.py b/openmc/settings.py
index 6774427554..16d4e9578d 100644
--- a/openmc/settings.py
+++ b/openmc/settings.py
@@ -6,7 +6,7 @@ from xml.etree import ElementTree as ET
 
 import openmc.checkvalue as cv
 from . import VolumeCalculation, Source, RegularMesh
-from ._xml import clean_indentation, get_text
+from ._xml import clean_indentation, get_text, reorder_attributes
 
 
 class RunMode(Enum):
@@ -1375,6 +1375,7 @@ class Settings:
             p /= 'settings.xml'
 
         # Write the XML Tree to the settings.xml file
+        reorder_attributes(root_element)  # TODO: Remove when support is Python 3.8+
         tree = ET.ElementTree(root_element)
         tree.write(str(p), xml_declaration=True, encoding='utf-8')
 
diff --git a/openmc/tallies.py b/openmc/tallies.py
index cf09c14161..646482ede9 100644
--- a/openmc/tallies.py
+++ b/openmc/tallies.py
@@ -14,7 +14,7 @@ import scipy.sparse as sps
 
 import openmc
 import openmc.checkvalue as cv
-from ._xml import clean_indentation
+from ._xml import clean_indentation, reorder_attributes
 from .mixin import IDManagerMixin
 
 
@@ -3143,5 +3143,6 @@ class Tallies(cv.CheckedList):
             p /= 'tallies.xml'
 
         # Write the XML Tree to the tallies.xml file
+        reorder_attributes(root_element)  # TODO: Remove when support is Python 3.8+
         tree = ET.ElementTree(root_element)
         tree.write(str(p), xml_declaration=True, encoding='utf-8')