The effect of substituent cation (Ca2+, Y3+ and Ce4+) on the stabilization of phase III and I in sodium sulfate is studied by Raman spectroscopy, conductivity and DSC measurements, and powder X-ray diffraction methods. Anomalies in the temperature dependence of the ionic conductivity data are correlated with thermal events in the DSC thermograms at temperatures where no phase transitions in the pure parent compound have been reported.