The present work investigates the processes of densification and grain growth of Ce 0.9 Gd 0.1 O 1.95Àd (CGO10) during sintering under reduced oxygen partial pressure. Sintering variables were experimentally characterized and analyzed using defect chemistry and sintering constitutive laws. Based on the results achieved, the grain size-relative density relationship, the densification rate and the grain-growth rate were determined. The activation energies for densification and grain growth were evaluated, and the dominant densification mechanism was indicated. For comparison, the densification behavior of CGO10 sintered in air was also studied. Accelerated densification was observed in early-stage sintering of CGO10 in a reducing atmosphere. This might be attributed to the oxygen vacancies generated by the reduction of Ce 4+ to Ce 3+ in the reducing atmosphere, which facilitate the diffusion of ions through the lattice. The densification activation energy of CGO10 in the reducing atmosphere was evaluated to be 290 ± 20 kJ mol À1 in the relative density range of 0.64-0.82, which was much smaller than that of CGO10 sintered in air (770 ± 40 kJ mol À1 ). The grain-growth activation energy of CGO10 sintered in the reducing atmosphere was evaluated to be 280 ± 20 kJ mol À1 in the grain size range of 0.34-0.70 lm. The present work describes a systematic investigation of sintering behavior of CGO10 under reduced oxygen partial pressure, which contributes to the first known determination of the fundamental parameters associated with densification and grain growth during early-stage sintering of CGO10 in a reducing atmosphere.