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 4-16 nm 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 degrees 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 bulk CuO in both isothermal and ramping reduction conditions, nanocrystalline CuO reduces to a stable Cu2O phase rather than forming metallic copper directly. The behavior of the CuO nanoparticles in temperature ramping reducing conditions is controlled by the particle size, with the smaller CuO nanoparticles exhibiting a greater stability and withstanding a higher temperature before their reduction to Cu2O and then to metallic copper nanoparticles. (c) 2006 Elsevier B.V. All rights reserved.