Evidence for adsorption assisted desorption of (NH3)-N-13 in the presence of gas phase ammonia on gamma-alumina is presented. Due to lateral interactions with physisorbed ammonia, the adsorption enthalpy of ammonia on gamma-alumina was measured to be -11 +/- 1 kJ/mol, using a (NH3)-N-13/(NH3)-N-14 isotopic exchange method. This adsorption/desorption exchange was monitored with positron emission pro. ling. Infrared spectroscopy showed ammonia to exchange preferentially on the Lewis acid sites of alumina.