The oxygen uptake ability of Pr-CeO2-based oxygen carriers, catalysts, and solid oxide fuel cells can be attributed to 3+ cation generation and the presence of vacant oxygen sites. Oxygen occupancies of CeO2, Pr-CeO2, and 5% Cu-doped Pr-CeO2 were investigated using neutron diffraction and related to the oxygen uptake as determined using thermogravimetric analysis (TGA). The presence of vacant tetrahedral oxygen sites at room temperature did not correspond to low-temperature oxygen uptake. The materials is did not uptake oxygen at 420 degrees C, but oxygen uptake was observed at 600 degrees C, which indicated that a minimum temperature needs to be met to generate sufficient vacancies/3+ cations. Variations in the lattice parameter as a function of temperature were revealed using in situ X-ray diffraction (XRD). With increasing temperature the lattice parameter increased linearly due to thermal expansion and was followed by an exponential increase at similar to 300-400 degrees C as cations were reduced. Despite segregation of Cu into CuO at high dopant concentration, at 600 degrees C a higher O-2 uptake was obtained for Ce0.65Pr0.20Cu0.15O2-delta (120 mu mol g(-1)), in comparison to Ce0.75Pr0.2Cu0.05O2-delta (92 mu mol g(-1)), and was higher than that for Ce0.8Pr0.2O2-delta (55 mu mol g(-1)). Both Pr and Cu introduce vacancies and promote the O-2 uptake of CeO2.