A kinetic study was made of the reaction of activated carbon with oxygen, uncatalysed and catalysed by MoO3, using a microbalance to record the loss of weight as a function of time. Orders and activation energies were measured. MoO3 was found to be a moderate catalyst for this reaction. The effect of loading on reactivity is linear for low loadings and shows saturation above 2.5 wt%. It is proposed that this type of reaction, involving three phases and two interfaces, can be explained by surface catalysis at the gas/catalyst interface with continuous diffusion of carbon atoms through the catalyst.