The interaction between gaseous oxygen and a catalyst Pt-Rh supported on gamma-Al2O3 (specific surface area: 100 m(2) g(-1)) was investigated by means of a differential desorption technique called intermittent temperature-programmed desorption (ITPD). Experiments were carried out under conventional secondary vacuum (P approximate to 10(-4) Pa). Essentially, three desorption steps alpha, beta(1) and beta(2) were observed, occurring around 350, 650, and 750 K, respectively. Steps beta(1) and beta(2) stem from the desorption of oxygen strongly and dissociatively adsorbed on the metallic particles. Desorption step alpha is better observed after oxygen adsorption at ambient temperature; it corresponds to weakly bonded oxygen. The high values of the frequency factors strongly suggest that no readsorption occurred when oxygen desorbed through steps beta(1) and beta(2). Desorption activation energies for steps beta(1) and beta(2) were estimated at 219 and 305 kJ/mol, respectively.