Ammonia decomposition on Ir(100) has been studied over the pressure range from ultrahigh vacuum to 1.5 Torr and at temperatures ranging from 200 to 800 K. The kinetics of the ammonia decomposition reaction was monitored by total pressure change. The apparent activation energy obtained in this study (84 kJ/mol) is in excellent agreement with our previous studies using supported Ir catalysts (Ir/Al2O3 82 kJ/mol). Partial pressure dependence studies of the reaction rate yielded a positive order (0.9 +/- 0.1) with respect to ammonia and negative order (-0.7 +/- 0.1) with respect to hydrogen. Temperature-programmed desorption data from clean and hydrogen co-adsorbed lr(100) surfaces indicate that ammonia undergoes facile decomposition on both these surfaces. Recombinative desorption of N-2 is the rate-determining step with a desorption activation energy of similar to 63 kJ/mol. Co-adsorption data also indicate that the observed negative order with respect to hydrogen pressure is due to enhancement of the reverse reaction (NHx + H --> NHx+1, x = 0-2) in the presence of excess H atoms on the surface.