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.