The technique of solid electrolyte potentiometry (SEP) was employed to monitor in situ the oxygen activity (the oxidation state) of La(Sr)MnO3 catalyst during CO oxidation under steady-state and transient conditions. The reaction at 550 degrees C was found to proceed through an ionic redox mechanism; the reoxidation and the reduction reactions occur at different sites on the catalyst surface and oxygen diffuses through the oxide lattice from the ox-sites, where it is adsorbed, to the r-sites, where it reacts with CO. Under reaction conditions it cannot be assumed that oxygen adsorbed at the r-sites is in equilibrium with oxygen adsorbed at the ox-sites, The rate was accordingly found to be a function of the catalyst oxygen activity and the gas-phase oxygen (reoxidation determined reaction) or carbon monoxide (reduction-determined reaction) partial pressure. Transient data can also be interpreted within the framework of the ionic redox model.