Heats of adsorption of dioxygen and oxidation phenomena of supported platinum metals (Pd, Pt and Rh) have been investigated by a simultaneous TG-DSC technique over a wide temperature range between 280 and 900 K. The oxidation proceeds in three consecutive stages, i.e. surface adsorption (T < 300 K), progressive penetration of adsorbed oxide ions into bulk (300-700 K), and complete oxidation to form stable oxides (700-900 K) for Pd/Al2O3 and Rh/Al2O3 catalysts. For Pt/Al2O3, in the same temperature range, the surface oxide was decomposed when T > 800 K. The crystallite sizes of supported platinum metals have a profound effect on the heats of adsorption (DeltaH(ad)), heats of stable oxide formation (DeltaH(f)) and formation of oxide species.