Two Pt electrodes with different morphologies were prepared and studied using an oxygen-ion conducting solid electrolyte electrochemical cell. Electrode 1 had a very porous catalyst/solid electrolyte interface and hence a high exchange current density while electrode 2 had a much less porous interface and consequently a lower exchange current density. Open-circuit reaction rate measurements showed that the effective Pt surface areas of the two electrodes were similar. Carbon monoxide oxidation at temperatures from 623 to 773 K was performed over the two electrodes. Under similar gas-phase conditions and at the same overpotentials electrode 1 showed a nonfaradaic modification of activity while electrode 2 did not (the current densities passed were very different due to a difference in the exchange current densities of the electrodes). This suggests that the work functions of the two electrodes were not modified to the same extent by a similar imposed overpotential. A model is suggested whereby the kinetics of electrochemical oxygen supply to (or removal from) the electrode surface must be relatively fast compared to the rate of decomposition and reaction of an electrochemically induced spillover species to observe rate modifications.