The reduction potential for the Col1-NH3-O2 system at a glassy-carbon 'electrode in a solid-state Nafion-coated cell is about 800 mV more positive than obtained for a glassy-carbon-Nafion film electrode when immersed in an aqueous solution containing Cot, ammonia, and dioxygen (+0.3 and -0.5 V vs. Ag/AgCI, respectively); this is attributed to two different electrode reactions: reduction of [(NH3)5Co-02-Co(NH3)5]4+ in the absence of a contacting liquid electrolyte solution and free O2 in aqueous Coil-ammoniacal solution.