A simple model to describe rate changes during the reversible electrochemical promotion of supported metal catalysts has been formulated. Polarisation of the interface between the ion-conducting support and the metal modifies the electron extraction potential of the metal as a result of the spillover of ionic oxygen species. By considering a simple reaction mechanism that involves a charged transition state and accounting for the effect of long-range electrostatic interactions, it is possible to express reaction rates as a function of applied overpotential. It is shown that under certain conditions reaction rates are predicted to increase on application of both negative and positive overpotentials. in agreement with some experimental studies (e.g. Refs. [Catal. Today I I (1992) 303 J, Catal. 159 (1996) 189]). as a result of electron donation and acceptance from both the metal catalyst and the surface O/O2- redox couple.