The oxidation of a mixture of benzene, toluene, 1-hexene and isooctane in the absence and presence of carbon monoxide has been studied over Pt/Al2O3 and Rh/Al2O3 monolith catalysts arranged singly and in various 1:1 and 4:1 combinations. Physical mixtures with both ratios are much more active than either single metal catalyst for complete removal of hydrocarbons when CO is present. The origin of the improvement is the high activity of Rh for oxidation of CO and hexene which allows the Pt to oxidise isooctane and benzene free of inhibition. Sequential beds in the order Rh then Pt are marginally more active than the corresponding mixed beds, but the order Pt then Rh is highly detrimental since the beneficial effect of prior or concurrent oxidation of CO over the Rh is lost.