X-ray photoelectron spectroscopy was used to study the effect of atomic oxygen on Ru(0001), and the effect of dissociated ammonia on RuO 2 /Ru(0001), in UHV conditions at ambient temperature. The Ru(0001) surface was exposed, at ambient temperature, to a mixed flux of atomic and molecular oxygen generated by dissociation of O 2 in a thermal catalytic cracker, with ∼45% dissociation efficiency. The detailed study of the XPS spectra shows the formation of a disordered multilayer oxide (RuO 2 ). No formation of higher oxides of Ru was observed. The formation of RuO 2 proceeded without saturation for total oxygen exposures of up to 10 5 Langmuir, at which point an average oxide thickness of 68 Å was observed. RuO 2 formed by the reaction with atomic oxygen was exposed to a flux of NH x (x=1, 2)+H generated by the cracker. The reduction of RuO 2 to Ru metal was observed by XPS. An exposure of 3.6×10 2 L of NH x +H, resulted in the observation of adsorbed H 2 O and OH, but no evidence of lattice oxide. The chemisorbed species were removed by additional NH x +H exposure. No nitrogen adsorption was observed.