The kinetics of H-2 oxidation by gaseous O-2 were investigated on a Pt-black catalyst film also serving as a working electrode in a Nafion 117 solid polymer electrolyte ceL1. The Pt-covered side of the Nafion 117 membrane was exposed to the flowing H-2-O-2 mixture and the other side was in contact with a 0.1 M KOH aqueous solution with an immersed Pt ounterelectrode. It was found that the applied current and the catalyst-electrode potential affect the catalytic rate of H-2 oxidation in a very pronounced and reversible manner. Positive currents enhance the rate of H-2 oxidation by up to 2000%. The increase in the rate of H, oxidation is up to 300 times larger than the electrochemical rate, I/2F, of H oxidation. This shows that the catalytic properties oi the Pt film are significantly modified by varying catalyst potential, as also manifested by the observed potential-induced change in the functional dependence of the catalytic rate on the partial pressures of H-2 and O-2. This novel application of solid polymer electrolytes to act as electrochemically activated catalyst supports, and thus promote and control heterogeneous catalytic reactions, is of considerable theoretical and possibly practical interest.