Gadolinium has been added to nuclear fuel to enable longer fuel cycles. UO2-Gd2O3 mixed fuel shows bad sintering behavior, which is hindered at temperatures of [approximately]1200°C. The mechanism that explains this unusual sintering behavior is not yet fully understood. The formation of Gd-rich phases with low diffusivity or pore formation during sintering have both been suggested as possible causes. Experimental data published in the literature appear not to support phase formation; however, the formation of large closed pores in Gd2O3-rich regions of the fuel has been reported. Pore formation could be developed during the Gd2O3 C[right arrow]B phase transition, which is followed by volume reduction. Our study investigated this hypothesis. The results showed that the mechanism proposed does not explain the UO2-Gd2O3 sintering behavior.