This paper describes the application of solid electrolyte potentiometry (SEP) to a perovskite catalyst, La0.5Sr0.5MnO3, with simultaneous measurements of the rate of carbon monoxide oxidation. The objective of the study was to examine the relationship between catalyst oxygen activity, as measured by SEP, and the performance of the catalyst for a typical oxidation reaction. The experimental study was confined to the temperature range 500-550 degrees C. at the lower temperature, the catalytic behaviour was shown to fit Mars-van Krevelen redox kinetics. The existence of hysteresis in both the reaction rate and the oxygen activity was demonstrated at the higher temperature. A novel approach was taken in developing a model to interpret this behaviour based on a dual-site redox model involving lattice oxygen diffusion (ionic redox model). The model was shown to correlate well with the observed data. These results have demonstrated, for the first time, that the reaction rate is a function of both gas phase composition and catalyst oxygen activity.