Identifying the composition of the catalytically active state of metal catalysts under dynamic operating conditions is of particular importance for oxidation catalysis. Here we report new insights into the chemical identity of different catalytically active states formed on a Ru(0001) catalyst during CO oxidation at various reaction temperatures. The changes in the surface composition of the Ru catalyst and the CO2 yield under varying reaction conditions in the 10(-4)-10(-1) mbar pressure range were followed in situ by synchrotron-based high-pressure X-ray photoelectron spectroscopy and mass spectroscopy. The results reveal that the catalytic activity of a few layers thick surface oxide without well-defined stoichiometry and structure is comparable with that of the stoichiometric RuO2(110) phase. This surface oxide forms under reaction conditions when RuO2 formation is kinetically hindered and can coexist with RuO2 in wide temperature and pressure ranges. (c) 2006 Elsevier Inc. All rights reserved.