Active sites responsible for the preferential oxidation of carbon monoxide were investigated using 4 wt.% Cu-CeO2 catalysts prepared by flame spray pyrolysis. Surface redox properties of the catalyst were assessed using a series of temperature-programmed reduction (CO, H-2 and mixed) experiments, as well as operando infrared spectroscopy. It was demonstrated that CO and H-2 react at identical surface sites, with CO2 formation proceeding simultaneously via three distinct Cun+-CO carbonyl species. The origin of high catalytic selectivity towards CO at below 150 degrees C stems from the carbonyl stabilization effect on the catalyst surface, preventing adsorption and subsequent oxidation of H-2. Under non-selective conditions at higher temperatures, a gradual red-shift and loss of intensity in the carbonyl peak was observed, indicating reduction of Cu+ to Cu-0, and the onset of an alternate redox-type oxidation mechanism where CO and H-2 compete for the oxidation sites. These results for Cu-CeO2 suggest that improved low-temperature catalytic activity will only be achieved at the expense of reduced high-temperature selectivity and vice versa. (C) 2010 Elsevier Inc. All rights reserved.