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In situ time-resolved X-ray diffraction (TR-XRD) using synchrotron radiation has been used to capture the dynamics of the reduction of nanocrystalline CuO using a normal supply of CO gas. Copper(II) oxide nanoparticles 4–16nm in width, as measured by XRD peak broadening, are synthesized using an aqueous organic-nitrate method and reduced in isothermal and temperature ramping reduction experiments. Temperature-programmed reduction of CuO nanoparticles using a ramping heating profile was observed to result in the sequential reduction process CuO→Cu₂O→Cu, with CuO reducing completely to the intermediate Cu₂O phase before further reduction to metallic copper. Isothermal reduction experiments at 250°C show that CuO nanoparticles completely reduce to Cu₂O, and this phase remains stable without further reduction with continued exposure to CO. In contrast to what is typically observed in …