The catalytic behaviour of the perovskite oxide La0.5Sr0.5MnO3-delta was investigated during carbon monoxide oxidation at 500-550 degrees C. The technique of solid electrolyte potentiometry was employed to monitor the oxygen activity of the solid catalyst under reaction conditions. At 550 degrees C, the reaction rate was found to be a function of the gas phase oxygen (or carbon monoxide) partial pressure and the catalyst oxygen activity. At 500 degrees C, the rate tended to depend only on the gas phase composition and not on the catalyst state. The results suggested that the reaction mechanism changes from an ionic redox model (550 degrees C), where the reoxidation of the reduced sites occurs by lattice oxygen, to a Mars-van Krevelen redox model (500 degrees C), where the reoxidation occurs by gas phase oxygen. The hysteresis at 550 degrees C was attributed to diffusional limitations appearing during either the reoxidation of the reduced sites or the reoxidation of the oxide bulk. The change in the reaction mechanism in going from 500 to 550 degrees C was considered to be the result of an increase in the mobility of lattice oxygen, in agreement with the oxygen temperature programmed desorption profile of the catalyst.