We have studied the oxidation of carbon monoxide over a lanthanum substituted perovskite (La0.5Sr0.5CoO3−d) catalyst prepared by spray pyrolysis. Under the assumption of a first-order kinetics mechanism for CO, it has been found that the activation energy barrier of the reaction changes from ∼80 to ∼40 kJ mol−1 at a threshold temperature of ca. 320 °C. In situ XPS near-ambient pressure (∼0.2 torr) shows that the gas phase oxygen concentration over the sample decreases sharply at ca. 300 °C. These two observations suggest that the oxidation of CO undergoes a change of mechanism at temperatures higher than 300 °C.