The heats of adsorption of N-2 and Ar on solid acids were compared by applying the Clausius-Clapeyron equation to isotherms. The heats on H-mordenite and H4SiW12O40 supported on SiO2 decreased gradually with increasing the adsorption amount in parallel with each other, while sulfated zirconia and Al2O3 gave large difference between N-2 and Ar in low adsorption range more than 30 and 15 kJ mol(-1), respectively, enabling the result to distinguish the Bronsted- and Lewis-acid types.