diff --git a/openmc/data/reaction.py b/openmc/data/reaction.py
index c5abc73c3..05b6b8d2d 100644
--- a/openmc/data/reaction.py
+++ b/openmc/data/reaction.py
@@ -894,10 +894,7 @@ class Reaction(EqualityMixin):
             Tgroup = group.create_group(T)
             if self.xs[T] is not None:
                 dset = Tgroup.create_dataset('xs', data=self.xs[T].y)
-                if hasattr(self.xs[T], '_threshold_idx'):
-                    threshold_idx = self.xs[T]._threshold_idx + 1
-                else:
-                    threshold_idx = 1
+                threshold_idx = getattr(self.xs[T], '_threshold_idx', 0)
                 dset.attrs['threshold_idx'] = threshold_idx
         for i, p in enumerate(self.products):
             pgroup = group.create_group('product_{}'.format(i))
@@ -939,7 +936,7 @@ class Reaction(EqualityMixin):
                             'at T={} because no corresponding energy grid '
                             'exists.'.format(mt, T))
                     xs = Tgroup['xs'][()]
-                    threshold_idx = Tgroup['xs'].attrs['threshold_idx'] - 1
+                    threshold_idx = Tgroup['xs'].attrs['threshold_idx']
                     tabulated_xs = Tabulated1D(energy[T][threshold_idx:], xs)
                     tabulated_xs._threshold_idx = threshold_idx
                     rx.xs[T] = tabulated_xs
diff --git a/src/reaction.cpp b/src/reaction.cpp
index 222822230..b733b2028 100644
--- a/src/reaction.cpp
+++ b/src/reaction.cpp
@@ -42,8 +42,6 @@ Reaction::Reaction(hid_t group, const std::vector<int>& temperatures)
     // Get threshold index
     TemperatureXS xs;
     read_attribute(dset, "threshold_idx", xs.threshold);
-    // TODO: change HDF5 format so that threshold_idx is 0-based
-    --xs.threshold;
 
     // Read cross section values
     read_dataset(dset, xs.value);