AbstractThe electric field has a large effect on the stoichiometry and grain growth of UO_2+x during Spark Plasma Sintering. UO_2+x is gradually reduced to UO_2.00 as a function of sintering temperature and time. A gradient in the oxidation state within the pellets is observed in intermediate conditions. The shape of the gradient depends unequivocally on the direction of the electrical field. The positive surface of the pellet shows a higher oxidation state compared to the negative one. An area with larger grain size is found close to the positive electrode, but not in contact with it. We interpret these findings with the redistribution of defects under an electric field, which affect the stoichiometry of UO_2+x and thus the cation diffusivity. The results bear implications for understanding the electric field assisted sintering of UO₂ and non-stoichiometric oxides in general.