In this paper an X-ray photoelectron Spectroscopy (XPS) study of the interaction between oxygen and Pt clusters on Teflon and on SiO2 supports is presented. Unambiguous experimental data give direct evidence of the oxidation of Pt clusters. PtOx appears to form spontaneously in conditions where, in contrast, a continuous Pt surface does not oxidize. It is also shown that the oxidation rate depends on the coverage and on the nature of the substrate. These results are interpreted in terms of the relative occupations of the Pt s and d orbitals, the effective coordination number of the Pt cluster atoms and the different shape of the clusters on Teflon or on SiO2 supports. © 1990.