A newly developed thermistor electrode was applied to determine the electrochemical molar Peltier heat and standard entropy of the insertion electrochemical reactions of solid copper(II) hexacyanoferrate and iron(III) hexacyanoferrate(II) (Prussian blue). The results were compared with those obtained with a thermostated cell. The study shows that the inserting ions, i.e. potassium and sodium ions, are mainly responsible for the entropy of the electrochemical reaction, whereas the nitrogen coordinated metal ions, i.e. copper(II) and iron(III) ions have no significant effect. The entropy effect associated with the inserting ions results from the partial dehydration upon insertion and hydration upon release of these ions, and also from the entropy contribution of the inserting ions themselves.