The relative acid strength and the number of acid sites of solid acids exhibiting high surface acidity have been determined from argon adsorption isotherms using Langmuir's equation in the temperature range T = 203–243 K and Henry's equation in the temperature range T = 233–313 K. The heat of Ar adsorption from Langmuir's equation was −14.9, −15.6, −17.9, −17.7, −17.1, −26.3, −29.6, −21.4, and −21.6 kJ mol−1 for silica–alumina, H–Y, H–ZSM–5, H–-MOR, and H–Beta zeolites, sulfated ZrO2, sulfated SnO2, tungstated ZrO2, and Pt-loaded tungstated ZrO2, respectively. The corresponding values of saturated adsorption amount were 0.35, 0.35, 0.37, 1.44, 0.10, 0.009, 0.10, 0.04, and 0.07 mmol g−1, respectively. The heat of Ar adsorption from Henry's equation was −14.4, −14.8, −17.3, −17.3, −18.9, −22.4, and −23.5 kJ mol−1 for silica–alumina, H–Y, H–ZSM–5, and H–MOR zeolites, sulfated Fe2O3, sulfated ZrO2, and sulfated SnO2, respectively. The order of the magnitude of heat for silica–alumina, the zeolites, and for the sulfated ZrO2 agree with those evaluated using Ar–TPD and NH3 –TPD, and coincide with the values of the heat of adsorption of NH3 . The saturated adsorption amounts for H–ZSM–5 and H–MOR are equal to the Al content, which generates acid sites, and approximate to the values for NH3 adsorption reported in the literatures.