The potentials of platinum oxidation according to the metallic particle size and the nature of the support have been evaluated. The use of the Fe3+/Fe2+ redox system potentials of platinum oxidation to be defined according to the dispersion of platinum and the nature of the support. By the use of the Gibbs-Helmoltz law (DeltaG = -zF Delta E), the comparison with bulk platinum allows the platinum excess free energy to be calculated. It thus appears that the platinum-support interaction decreases following the sequence: Al2O3 > C > SiO2. However, the most important parameter seems to be the size of the particle. Thus, the small particles are oxidised more easily by the Fe3+/Fe2+ redox system. At the end of the process, the size of the most stable particles is defined by a compromise between a decrease of the stability when the dispersion increases and an increase of the metal-support interaction when the particle size decreases.