The oxidation of 5 and 8 nm palladium particles supported on SiO2/Si(100) has been studied with XPS. During oxidation the thickness of the oxide layer increases linearly with time. The lattice rearrangement needed for the formation of a new oxide layer at the metal-oxide interface is probably the rate-determining step. There were no significant differences between the oxidation of the 5 and 8 nm particles. The rate of the oxidation is strongly temperature-dependent. The activation energy for the oxidation is at least 100 kJ/mol. For comparison the oxidation and reduction of a 8 wt% Pd/SiO2 catalysts was studied. The results indicated that oxidation and reduction of the Pd/SiO2 catalyst proceeds in a similar way as on the Pd/SiO2/Si(100) model catalysts.