An ac electrochemical thermal method with a polyvinylidene difluoride (PVDF) pyroelectric film sensor and the lock-in amplifier technique was developed for determining the entropy change Delta S of an electrode reaction. It was found that the Peltier heat could be determined separately from the polarization heat and Joule heat by use of a lock-in amplifier with the reference frequency the same as the frequency of the ac current. For two reversible electrode reactions the Delta S value for each electrode could be calculated by measuring the amplitudes of the ac electrochemical thermal signal V-0 ac current I-0, the phase shift between them and concerning the total entropy change of the whole cell reaction. The Fe(CN)(6)(4-)/Fe(CN)(6)(3-) electrode and its Delta S value could be used as a calibration rule to measure the Delta S for other single electrode reactions. The Delta S values for the reduction reactions in the systems of Fe2+/Fe3+, Cu/Cu2+ Br-/Br-2 and H-2/H+ were determined. The reaction mechanisms for Cu/Cu2+ and Br-/Br-2 electrodes were discussed from the view of entropy changes. Comparing with conventional and absolute ionic entropy systems, it was found that the conventional system is more convenient for practical use.