Ammonia adsorption on the Pt(111) and stepped Pt(S)-6(111)×(111) surfaces has been studied using thermal desorption spectroscopy, isotope exchange thermal desorption, and steady-state decomposition measurements. Molecular ammonia adsorbs without dissociation on both the Pt(111) and Pt(S)-6(111)×(111) surfaces below 400 K. Three distinct desorption peaks are observed following adsorption at 90 K. Multilayers of solid ammonia desorb in a narrow peak at 100 K. The weakly chemisorbed high coverage–low temperature form of molecular ammonia desorbs at 150 K with first order kinetics and a heat of desorption of 36 ± 3 kJ/mol (8.6 ± 0.6 kcal/mol). The chemisorbed low coverage–high temperature form of molecular ammonia desorbs in a broad peak in the 170 to 450 K temperature range. A small amount of adsorbed molecular ammonia (<1% of saturation coverage) undergoes isotope exchange with preadsorbed deuterium. These results indicate that exchange occurs via an associative process, probably involving an NH3D type intermediate species. Steady-state ammonia decomposition occurs above 400 K on the Pt (111) surface indicating that dissociative adsorption does occur at elevated temperatures.