Polychlorinated benzenes (PhClx) are formed as byproducts in the combustion of chlorobenzene on Pt supported on gamma-Al2I3, SiO2, SiO2-Al2O3, or ZrO2. The congener and isomer distribution of the PhClx differs for the various supports. The amounts of PhClx correlate with the dispersion of platinum. Thus, a Pt/gamma-Al2O3 catalyst calcined at 500 degrees C to yield very small Pt crystallites was more active in PhClx formation than Pt/gamma-Al2O3 calcined at 800 degrees C. In all cases T-50% for chlorobenzene conversion is close to 300 degrees C and appears to be independent of the crystallite size of the platinum. Replacing platinum by palladium led to lower rates of combustion and to more byproducts. These results lead us to propose that, in the presence of Cl and higher oxygen concentrations, small Pt crystallites are converted more easily into Pt(IV) species. These are less efficient in combustion, but can be more active in chlorination. (C) 2000 Elsevier Science B.V. All rights reserved.