High-resolution x-ray photoemission spectroscopy (XPS) was used to examine the surface composition and chemical bonding of calcined UO2 powders as a function of. process parameters. It is believed that the surface composition of the powder grains ultimately affects packing density. XPS quantitative analysis revealed O/U ratios indicative of mixed uranium valences. In addition, high-resolution U4f(7/2,5/2) core-level spectra revealed reoxidation of U4+ to U6+ for the lower temperatures, and a reduction of U6+ to U4+ for the higher temperatures using 4% H-2/Ar. Reoxidation can also be affected by the presence of water or OH, and the powder samples calcined at the lower temperatures have the highest OH-/O2- peak area ratio. The valence-and electronic structure for the higher-temperature calcinations clearly shows a stronger 5f emission peak near the Fermi edge, a feature that is indicative of a highly localized state. Bulk structural analysis using x-ray diffraction also indicates a number of uranium oxide phases produced by calcination. However, at 600 degreesC the UO2 cubic phase resulted. (C) 2003 American Vacuum Society.