diff --git a/torax/config/plasma_composition.py b/torax/config/plasma_composition.py
index ce133f85..4dfb1111 100644
--- a/torax/config/plasma_composition.py
+++ b/torax/config/plasma_composition.py
@@ -19,6 +19,7 @@
 from collections.abc import Mapping
 import dataclasses
 import logging
+import typing
 
 import chex
 import numpy as np
@@ -61,6 +62,15 @@ def __post_init__(self):
     if not isinstance(self.species, Mapping):
       raise ValueError('species must be a Mapping')
 
+    # Iterate through species keys and check if they are in the allowed list.
+    allowed_symbols = typing.get_args(constants.ION_SYMBOLS)
+    for ion_symbol in self.species:
+      if ion_symbol not in allowed_symbols:
+        raise ValueError(
+            f'Invalid ion symbol: {ion_symbol}. Allowed symbols are:'
+            f' {allowed_symbols}'
+        )
+
     time_arrays = []
     fraction_arrays = []
 
diff --git a/torax/config/tests/plasma_composition.py b/torax/config/tests/plasma_composition.py
index 3a42014c..333c5c23 100644
--- a/torax/config/tests/plasma_composition.py
+++ b/torax/config/tests/plasma_composition.py
@@ -147,6 +147,7 @@ class IonMixtureTest(parameterized.TestCase):
       ('valid_tolerance', {'D': 0.49999999, 'T': 0.5}, False),
       ('invalid_tolerance', {'D': 0.4999, 'T': 0.5}, True),
       ('invalid_not_mapping', 'D', True),
+      ('invalid_ion_symbol', {'De': 0.5, 'Tr': 0.5}, True),
   )
   def test_ion_mixture_constructor(self, input_species, should_raise):
     """Tests various cases of IonMixture construction."""