A simple model to describe the reversible electrochemical promotion of supported metal catalysts has been formulated. This model is derived from first principles and allows for the presence of two forms of catalyst surface oxygen, one ionic and one neutral. Both forms of surface oxygen are able to undergo charge-transfer reactions with the ion-conducting support. The ionic oxygen is assumed to be the only ion present with a significant surface coverage and, as such, dictates the properties of the catalyst surface double layer. Using this basis thermodynamic relationships governing the chemical potentials of the catalyst surface oxygen species are derived for the case of a pure ion-conducting support with fast surface and gas-phase diffusion of species. It is shown that, in the case of very stable oxygen ions, the application of a moderate overpotential to the catalyst/support interface results in a change in the catalyst surface potential equal to the applied overpotential provided that lateral interactions between oxygen ions can be neglected.