Up to 0.15 formula unit (f.u.) of U4+ incorporated in the Zr site of zirconolite by firing CaUxZr(1-x)Ti2O7 compositions in argon at 1400degreesC allows retention of the 2M polytype. Further U4+ substitution for Zr, up to 0.4 f.u., produces the 4M polytype (containing similar to0.4 f.u. of U) plus the 2M polytype containing 0.15 f.u. of U. The pyrochlore structure (containing 0.6 f.u. of U) forms in conjunction with the 4M polytype at U contents of 0.4 f.u. up to 0.7 f.u. Higher U contents give the pyrochlore structure, but the solid-state reactivity of even alkoxide-based preparations becomes increasingly poor for x > similar to0.7 and hot pressing in graphite dies at similar to1250degreesC is necessary to achieve near single-phase pyrochlore structures for x = 1. When samples of CaUxZr1-xTi2O7 stoichiometry (x = 0.1 and 0.2) are oxidized at 1400degreesC in air, diffuse reflectance spectroscopy (DRS) shows evidence for U5+ formation at the expense of U4+ via enhanced absorption bands, similar to50 nm in half-width, near 970 and 1500 nm and correspondingly weakened absorption at 1150 and 1660 nm. Weight gains consistent with complete oxidation of U4+ to U5+ are observed when finely powdered argon-fired samples with x = 0.1 and 0.4 are heated in air to 1200degreesC. Evidence for U valence states higher than +4 in both argon- and air-heated materials containing charge compensators to encourage U5+ or U6+ formation was also derived from DRS (showing U5+ in particular), and XANES. DRS shows weak absorption bands attributable to U4+ in zirconolites containing similar to0.2 f.u. of U incorporated in the Ca site via Mg or Al substitutions in the Ti sites, with the spectrum being closely but not exactly similar to that attributed to U4+ in the Zr site. Zirconolite and pyrochlore compositions which are melted at 1500degrees or 1550degreesC in argon and furnace-cooled yield broadly similar phase assemblages to the corresponding sintered materials, but there is evidence of incongruent melting in all materials.