The effect of oxygen on the nature, population, and reactivity of surface species formed during reduction of NO by CO over Rh/TiO2 catalysts has been examined employing FTIR and transient RIS techniques. It has been found that the activity of Rh is hindered by accumulation of surface oxygen originating from NO decomposition and gas-phase oxygen in the feed. Adsorbed CO and reduced TiO2-x species in the vicinity of Rh particles act as oxygen atom scavengers and, under fuel-rich conditions, remove atomic oxygen from the surface and restore the catalytic properties. Results of the present study provide additional evidence that production of N-2 is related to dissociation of adsorbed Rh-NO- while production of N2O is related to the presence of Rh(NO)(2). The presence of reduced Rh-0 sites is necessary for the formation of both reduction products. In the absence of oxygen in the feed, surface isocyanate species are also observed under reaction conditions. Their formation requires the presence of adjacent Rh-0-CO and reduced Rh-0 sites. Although these species are favored under conditions in which NO conversion to reduction products is observed, there is no evidence that they are catalytically active species. (C) Academic Press.