The adsorption and reaction of NH3 was studied on the Ru(11 (2) over bar0) surface using high-resolution electron energy loss spectroscopy and thermal desorption spectroscopy. Following NH3 exposure at 85 K, annealing to 140 K leads to desorption of the multilayer and second layer leaving the NH3 monolayer which is characterized by two adsorption states: alpha(1)-NH3 at a 3-fold hollow site and a more weakly bonded alpha(2)-NH3 at an on-top site. As the temperature is increased, the alpha(2)-NH3 desorbs molecularly, while partial dehydrogenation occurs for the alpha(1)-NH3. This process proceeds at 3-fold hollow sites in successive steps finally leading to atomic nitrogen. The bonding competition effect accounts for the low N coverage of 0.11 ML obtained following NH3 exposure at 85 K and annealing. The activation barrier for associative desorption of nitrogen is estimated to be 120 20 kJ/mol. At 300 K, the anticipated NH3 synthesis from successive hydrogenation of adsorbed N is observed.