The influence of Gd-doping level and oxygen stoichiometry on the structural properties and electrochemical reactivity of U-1 yGdyO2 +/- x have been investigated. The stoichiometry of UO2 matrices with different Gd contents was determined using the lattice parameter obtained by XRD. The extent of lattice contraction, defined by a contraction factor, was found to be dependent on the stoichiometry. The surface morphologies exhibited differences in grain size which varied with stoichiometry and its influence on the U atom diffusivity during fabrication. The differences in grain size and, hence, the density of grain boundaries was reflected in variations in electrical conductivity, with hyperstoichiometric specimens with a low number of boundaries, yielding an increase in conductivity with increasing Gd content. Cyclic voltammetry showed that a variation in Gd content had only a minor influence on the electrochemical reactivity of stoichiometric U-1 yGdyO2. By contrast, the reactivity of hypostoichiometric U-1 yGdyO2-x and hyperstoichiometric U-1 yGdyO2+x increased and decreased, respectively, with increasing Gd content. The formation of Gd-Ov clusters in hyperstoichiometric U-1 yGdyO2+x has a more marked influence on reactivity than the accompanying changes in grain size and electrical conductivity. (C) 2017 Elsevier Ltd. All rights reserved.