The acidities of gamma-Al2O3, HNa-Y zeolite, and H-mordenite have been examined by microcalorimetric measurements of ammonia adsorption at 423 K. The differential heat of adsorption on gamma-Al2O3 decreases continuously with ammonia coverage from an initial value of 165 kJ/mol at low coverages to a value of 70 kJ/mol at higher coverages. The differential heat of adsorption on HNa-Y zeolite shows similar behavior, with a plateau of nearly constant heat at 115 kJ/mol. H-mordenite exhibits a nearly constant heat of adsorption equal to 155 kJ/mol. The results from these microcalorimetric measurements are in agreement with thermogravimetric and temperature-programmed desorption results collected at higher temperatures. Adsorbed ammonia has sufficient mobility at 423 K to equilibrate with the catalyst surface on the time scale of microcalorimetric measurements, and these measurements provide an effective method for quantifying acid site distributions of solid-acid catalysts.