Rhodium particles in nanometer size were prepared by impregnating alumina powders with aqueous solutions containing rhodium salts. The dispersion (D) of rhodium crystallites on the prepared samples was estimated by dioxygen adsorption measured at 300 K. Phenomena of oxidizing the supported crystallites with 2.5 10 4 Pa O 2 in a temperature range between 280 and 870 K were calorimetrically studied. Extent of oxidation may be distinguished into three stages, i.e., adsorption on surface (T < 300 K), progressive penetration into bulk and formation of a stable bulk oxide (T > 700 K), on raising the oxidation temperature. Heat of dioxygen adsorption varies only slightly with the dispersion (D) of rhodium and has a value of 294±6 kJ (mol O 2 ) - 1 . Chemical stoichiometry of the bulk oxide formed however, varies with the dispersion of rhodium crystallites. A dioxide (RhO 2 ) (Δ f H = 225±3 kJ (mol O 2 ) - 1 ) and a sesquioxide (Rh 2 O 3 ) (Δ f H = 273±3 kJ (mol O 2 ) - 1 ) was formed at D < 60% and D > 80%, respectively.