The relative acid strength of sulfated SnO2, sulfated ZrO2, silica-alumina, H-Y, H-ZSM-5, and H-mordenite was estimated by the heat of Ar adsorption, calculated from a temperature dependence of amount of Ar adsorption around room temperature. These were 23.5, 23.6, and 13.8-16.5 kJ mol(-1) for sulfated SnO2, sulfated ZrO2, and H-type zeolites, respectively. The data indicate that Ar is suitable for a probe molecule of acid sites on solid surface and that this technique is useful for evaluation of the acid strength of solid superacids.