Microcalorimetric and infrared spectroscopic studies of ammonia and carbon dioxide adsorption have been used to study the effects on the acid/base properties of adding tin oxide to gamma-Al2O3. The addition of SnO2 to gamma-Al2O3 decreases the number of strong acid sites (heats of ammonia adsorption higher than 140 kJ/mol), increases the number of weaker acid sites (heats from 110 to 130 kJ/mol), and decreases slightly the number of basic sites (heats of carbon dioxide adsorption from 70 to 150 kJ/mol). In contrast, the presence of SnO on gamma-Al2O3 decreases the total number of acid sites (heats of ammonia adsorption higher than 70 kJ/mol) and eliminates most of the basic sites. Infrared spectroscopy of adsorbed ammonia reveals interactions between aluminum cations and stannous ions, leading to a decrease in the strength of the Lewis acid sites associated with aluminum cations.