A series of (U1-yGdy)O-2 materials (y = 0, 0.01, 0.03, 0.05, 0.07 and 0.10) were characterized by X-ray diffractometry and Raman spectroscopy to determine the influence of Gd-III doping on the structure of (U1-yGdy)O-2 solid solutions. The XRD results show that, while the fluorite structure is maintained, Gd doping up to 10% leads to a contraction of the fluorite lattice. Raman spectroscopy shows Gd-III doping distorts the fluorite lattice structure due to the formation of oxygen vacancies (O-v) and, possibly, MO8-type complexes, as a consequence of the differences in both the oxidation state and the ionic radius of Gd3+ compared to that of the U4+. The influence of Gd doping on the electrochemical reactivity of the (U1-yGdy)O-2 specimens was shown to be minor, possibly due to a competition between the increase in the number of Ov and the contraction of the lattice. (C) 2017 Elsevier Ltd. All rights reserved.