Ab initio calculations using density functional theory within the generalized gradient approximation have been performed for the interaction of oxygen with a reduced SnO2 (110) surface. We found two species of chemisorbed O-2 that can be described as a peroxo (O-2(2-)) species for side-on type adsorption and a superoxo (O-2(-)) species for end-on type adsorption on the basis of their geometries and vibrational frequencies. We also found a low dissociation barrier of 2.0 kcal/mol for the peroxo species and high migration barriers of similar to 13-18 kcal/mol for an oxygen atom produced by dissociation of O-2 on the reduced SnO2 surface. An analysis of the electronic density of states indicates that an adsorbed oxygen atom coupled with the nearest-neighboring bridging oxygen vacant site is an O- species that displays catalytic activity on the surface.