Catalytic oxidation of monochlorobenzene (667 ppm) in wet air was investigated over PtHFAU(5) catalysts, differing by their Pt content (from 0 to 1.1 wt.%), their Pt dispersion (for identical Pt content) and the electronic state of Pt. PtHFAU(5) catalysts show a higher activity compared to more conventional PtAl2O3 and PtSiO2 samples. For a given temperature, chlorobenzene oxidation over PtHFAU(5) is independent of the platinum particles size. On the other hand, a plateau in activity is reached from 0.4 to 0.6% Pt. Amount of polychlorinated benzenes (PhCl2+) Were produced in the order PtAl2O3 > PtSiO2 > PtHFAU(5). The formation of these compounds, especially PhCl2, over PtHFAU(5) was function on the Pt content (PhCl2 isomers appear only from 0.6% PtHFAU), and of the electronic state of Pt. Thus PhCl2, were mainly found over reduced Pt-0 particles certainly through a chlorination of platinum (formation of PtCl. species). A reaction scheme for the PhC12 formation was proposed. (c) 2005 Elsevier B.V. All rights reserved.