The phenomenon of electrochemical promotion of catalysis (EPOC) or non-Faradaic electrochemical modification of catalytic activity (NEMCA effect) with O-2(-) conductors is analyzed from a thermodynamic point of view. It is shown that thermodynamics dictates the existence of two oxygen species on the catalyst-electrode surface, one which can, under certain conditions, be equilibrated with the solid electrolyte, the other having a chemical potential not exceeding that of oxygen in the gas phase. The chemical potential of the former type of oxygen is controlled by the applied potential. These conclusions are in agreement with electrochemical, surface spectroscopic and STM investigations. (C) 2003 Elsevier B.V. All rights reserved.