In situ near edge X-ray absorption fine structure (NEXAFS) spectroscopy and ambient pressure X-ray photoelectron spectroscopy (AP-XPS) were performed to monitor the oxidation state and structure of 4 nm CoPt nanoparticles during the reaction of CO with O-2 - a model oxidation reaction. In addition, reversible changes in the oxidation state of cobalt as a function of cycling CO and O-2 pressure (in the range of millitorr to 60 Torr) were quantified and compared. Turnover frequency reaction data was also obtained for the CoPt nanoparticles and correlated with the oxidation states and structures observed spectroscopically. These findings indicate that separated from the effect of partial pressure of the reactant gases, chemical state and structure changes of the CoPt nanoparticles during CO oxidation are important factors in determining the rate of the reaction. (C) 2011 Elsevier B.V. All rights reserved.