The reaction of oxygen gas with CO adsorbed on two samples of Pt SiO2 with different particle sizes has been followed by direct monitoring of the infrared absorption near 2075 cm-1. In contrast to earlier reports the rate of CO2 formation did not show a simple second-order dependence on CO coverage and this discrepancy is explained by improved spectrometer performance. During the course of reaction the peak frequency of the residual CO molecules remained almost constant indicating the maintenance of high local CO coverage throughout. This is interpretable in terms of island formation on individual particles with reaction only at the interface between islands. Evacuation halted oxidation even though the sample as a whole still contained both adsorbed oxygen and adsorbed CO. Reaction between them became impossible probably because individual particles contained only one adsorbed species. An accelerated rate was observed when oxygen was readmitted to a partially reacted system. Over a limited range the fall in θCO was linear with ln(time) pointing to a steadily rising activation energy. Reaction of CO with an initially oxygen covered surface exhibited similar behavior whilst the presence of both gases led to maximal CO coverages below 100 °C even when oxygen was in excess. The origin of the increase in activation energy is believed to be the intrinsic inter- and intraparticulate heterogeneity of supported platinum.