An Ir-in-ceria catalyst has been developed, in which most of the iridium particles are embedded in the ceria matrix through the redox reaction between Ce3+ and Ir4+, which occurred during co-precipitation (CP). This Ir-CP catalyst exhibited high activity for preferential CO oxidation under excess hydrogen conditions, and the selectivity to CO2 remained nearly constant, at around 70%, with increasing reaction temperature. Temperature-programmed reduction and in situ diffuse reflectance infrared spectroscopy techniques were used to explore the structure of the Ir-CP catalyst and to correlate it with the catalytic performance. It was found that the CeO2 support was activated by iridium and formed on the surface the active sites for CO oxidation. Due to the absence of extensively exposed Ir species on the Ir-CP catalyst, H-2 oxidation occurring on the Ir species and the ceria support at high temperatures was significantly suppressed, thus keeping the selectivity to CO2 at a high level. (C) 2008 Elsevier Inc. All rights reserved.