The oxidation of toluene and benzene in a four hydrocarbon mixture with and without carbon monoxide present has been investigated over Rh/Al2O3 and Rh/CeO2/Al2O3 catalysts containing residual chlorine. Up to 12% of toluene can be converted to benzene with a near stoichiometric feed containing carbon monoxide. Much less benzene is formed in the absence of carbon monoxide, especially with the ceria-alumina based catalyst which is more active for the oxidation of the aromatics. Formation of benzene probably proceeds within areas where toluene coverages are sufficiently high to provide the hydrogen atoms required for addition to a benzyl intermediate. With fresh catalysts chlorobenzene can be formed as a byproduct in concentrations up to 10% of that of the aromatic in the feed. Toluene is the largest source of chlorobenzene with the alumina-based catalyst when CO is present. Benzene is a larger source with Rh/CeO2/Al2O3 when the feed is dry but with water present the behaviour resembles that of Rh/Al2O3. The processes involved in chlorobenzene formation are uncertain but may include migration of chlorine from alumina to rhodium or the metal-support interface.