Using supersonic molecular beam techniques we have investigated the dissociative adsorption of NH3 on a Ru(0001) surface. At high incident energies, the dissociation increases substantially due to a direct breaking of the N-H bond on impact with the surface. For low incident translational energies, the dissociation depends on surface temperature T-s in an unusual manner, peaking sharply around 400 K. Increasing the surface defect density by low-fluence Ar+ sputtering strongly enhances the dissociation probability while preserving the overall T-s-dependence. We interpret the low incident energy behavior as due to a mechanism in which a molecular precursor must undergo diffusion to defects before dissociating. At the lowest surface temperatures, dissociation is limited by the diffusion of the reaction products away from the defects in order to reactivate them. A kinetic model based on this mechanism is developed which is in good agreement with all experimental observations.