The use of a potentiometric sensor to monitor the state of an La0.5Sr0.5MnO3-delta catalyst during CO oxidation at 550 degrees C was investigated. The technique of solid electrolyte potentiometry was employed to monitor in situ the oxygen activity (the oxidation state) of the catalyst. The reaction proceeded through an ionic transport redox mechanism where the reoxidation of the reduced sites occurs by lattice diffusion of oxygen rather than by reaction with gas-phase oxygen. The rate was accordingly a function of the catalyst oxygen activity and the gas-phase oxygen (reoxidation determined reaction) or carbon monoxide (reduction determined reaction) partial pressure. Hence, the EMF of the sensor is necessary to predict the reaction rate for this system. Such a sensor could find wider application for reactor control, particularly for partial oxidation processes.