The kinetic isotope effect for combustion of a C6H5Cl/C6D5Cl mixture on Pt/gamma -Al2O3 was found to be close to unity between 520 and 580 K. However, in the presence of an excess of heptane, an isotope effect of 1.5 was found between 460 and 490 K. For the combustion of a C6H6/C6D6 mixture the k(H)/k(D) value was around 2 between 404 and 439 K. The results show that in the combustion of chlorobenzene per se, C-H bond activation is not a rate-determining step. On Pt sites, C-Cl bond scission probably occurs already at low temperatures. The chlorine and the phenyl group cannot easily react further. Chlorine on the surface is active in chlorination, which is shown by the formation of C6D5Cl in an experiment with C6H5Cl and C6D6. Only at a certain temperature is the chlorine removed, partly as polychlorinated benzenes. The removal of chlorine from the catalyst allows oxygen to take part in the reaction, which determines the rate of the combustion of chlorobenzene. When heptane is present, CI is removed from the surface and C-H bond scission can become rate determining, as is also the case in the combustion of C6H6/C6D6. Upon (partial) combustion of C6H6Cl/C6D5Cl and C6H6/C6D6 mixtures on a Pt/gamma -Al2O3 catalyst, hydrogen-deuterium exchange occurs on the gamma -Al2O3 support.