The preferential oxidation (PROX) of carbon monoxide to carbon dioxide in excess H-2 is one of the strategies for obtaining high purity H2 streams, especially for PEM fuel cells. A CoOx/ZrO2 catalyst was synthesized and studied for the PROX reaction under various reaction conditions and indicated that in the temperature window of interest (80-200 degrees C), this catalyst had potential for obtaining high conversions of CO with high 02 selectivity to CO2. Increases in the GHSV and the CO/O-2 ratio led to lower CO conversion but higher 02 selectivity to CO2. High temperature operation led to a decrease in CO oxidation activity and the formation of methane, which was attributed to partial reduction of CoOx/ZrO2. The catalyst performance was examined using steady-state and transient temperature-programmed reaction (TPRxn) experiments. Temperature-programmed reduction (TPR) and time-on-stream studies were used to examine the catalyst stability in reducing conditions. DRIFTS studies during CO TPD and in situ PROX were used to examine surface species on ZrO2 and CoOx/ZrO2. (c) 2007 Elsevier B.V. All rights reserved.