Oxygen-ion current pumping experiments were performed on a platinum electrode/catalyst in a solid-electrolyte electrochemical cell operating at atmospheric pressure and a temperature of 688 K. The electrode overpotential, extraction potential (through the use of a Kelvin probe), and reaction rate were all monitored during the experiments. At steady state, the Pt catalysts failed to exhibit the direct equality between the extraction potential and applied overpotential previously proposed by Vayenas et al. [Catal. Today, 11, 303 (1992)] and it was necessary to use the more general relationship that applied overpotential is equal to the change in extraction potential plus the change in outer potential. The extraction potential change, and concomitant reaction modification, appears to be kinetically determined by the relative rates of electrochemical oxygen spillover and removal. a linear relationship between the change in work function and the normalized change in reaction rate was found to describe accurately the behavior of the catalyst. This is in agreement with previous work by Vayenas et al. and indicates the presence of a common promotional mechanism, as expected.