[en] Full text: Applications of CeO₂ based materials in solid oxide fuel cell, catalyst and oxygen separation technologies require high oxygen storage capacities (OSC), for which the O₂ uptakes are largely dependent on the concentration of oxygen vacancies. Neutron diffraction carried out at the Australian Centre for Neutron Scattering (ECHIDNA instrument) was used to determine the oxygen occupancies of CeO₂, Pr-CeO₂ and 5% Cu-doped Pr-CeO₂, and related to the oxygen uptake measured using thermal gravimetric analysis. Refinement was carried out by allowing the formation of Frenkel defects in CeO₂ (Fm3m) via movement of the 8c tetrahedral oxygen anions to the 48i interstitial site. The 5% Cu doped Pr-CeO₂ material possessed lower oxygen occupancy than Ce_0.85Pr_0.15O_2-δ, and CeO₂ had the highest oxygen occupancy. This work shows that the formation of vacancies is encouraged by the addition of Pr to CeO₂ and further increased with Cu. Oxygen occupancies could not be determined for the 10% Cu and 15% Cu materials due to segregation. Evidence of vacant tetrahedral sites did not correlate with oxygen uptake at 420 °C, indicating that a minimum temperature is required to generate sufficient vacancies/+3 cations. At 600 °C, 15% Cu doped Pr-CeO₂ had the highest oxygen uptake (120 μmol.g^-1) followed by 5% Cu doped Pr-CeO₂ (92 μmol.g^-1) then Ce_0.8Pr_0.2O_2-δ (55μmol.g^-1). Cation reduction commenced at ~300–400 ℃ as shown through an exponential increase in the lattice parameter determined by in-situ X-ray diffraction. Our results show that both Pr and Cu introduce vacancies and promote the O₂ uptake of CeO₂ that is necessary for the development of high OSC materials. (author)