Charge transfer processes have been studied in the Ag/HAP/Pt system in an oxygen atmosphere. It has been found for the Ag/HAP/Pt contact that at negative potentials on Ag, molecular oxygen reduction currents are observed, which are limited by O-2 diffusion to silver. Anode reaction on silver is a formation reaction of Ag+ ions which accumulate near the interface by ion-exchange. At negative potentials on Pt, molecular oxygen reduction currents were also observed; at positive potentials on Pt, there was an oxygen evolution due to crystal water decomposition. The U voltage value generated in the Ag/HAP/Pt system at an O-2 pressure of 1 atm and 18 degrees C was 0.48 V. Increase in U values with rising temperature is attributed to the temperature dependence of the overpotential of oxygen reduction on Pt.