diff --git a/interfaces/matlab/toolbox/@ThermoPhase/nAtoms.m b/interfaces/matlab/toolbox/@ThermoPhase/nAtoms.m
index 154673be2..30e7901ad 100644
--- a/interfaces/matlab/toolbox/@ThermoPhase/nAtoms.m
+++ b/interfaces/matlab/toolbox/@ThermoPhase/nAtoms.m
@@ -1,6 +1,12 @@
 function n = nAtoms(a,k,m)
 % NATOMS - Number of atoms of element m in species k.
 if nargin == 3
+    if ischar(m)
+        m = elementIndex(a,m);
+    end
+    if ischar(k)
+        k = speciesIndex(a,k);
+    end
     n = phase_get(a.tp_id,14,k,m);
 else
     error('usage: nAtoms(phase, k, m)')