diff --git a/rmgpy/cantherm/input.py b/rmgpy/cantherm/input.py
index 78cbf21339e..66151c9e4b8 100644
--- a/rmgpy/cantherm/input.py
+++ b/rmgpy/cantherm/input.py
@@ -130,6 +130,13 @@ def species(label, *args, **kwargs):
         if structure:
             spec.molecule = [structure]
         spec.conformer = Conformer(E0=E0, modes=modes, spinMultiplicity=spinMultiplicity, opticalIsomers=opticalIsomers)
+        if molecularWeight is None:
+            if spec.conformer.number is None:
+                raise ValueError('No molecularWeight entered for species {0}, and it cannot be reconstructed from'
+                                 ' conformer.number'.format(spec.label))
+            else:
+                molecularWeight = Quantity(sum([getElement(int(element)).mass
+                                                for element in spec.conformer.number.value_si]), 'kg/mol')
         spec.molecularWeight = molecularWeight
         spec.transportData = collisionModel
         spec.energyTransferModel = energyTransferModel