Phase relations and defect structures in (U_o.1Th_o.9)0₂-Gd0_1.s quaternary system were studied at 1673 K. Different phases and their compositions. obtained on heating the oxides in different atmospheres were examined by powder X-ray diffraction and chemical analysis. respectively. The region of existence of fluorite type solid solutions was determined. The cubic lattice parameter of the solid solution. [(U_o.1Th_o.9)_1-yGd_y]0_2+^.1C, in single phase region (0 ≤ y ≤ 0.4) could be expressed in a linear equation of x and y as: a_o (A) = 5.581 - 0.228x - 0.198y for x < 0 O/U analysis and simple ionic model of the quaternary solid solutions indicate the oxidation ofU⁺⁴ to a mixture of U^+s and U⁺⁶• High temperature XRD studies of FCC structured [(U_O•1Th_o.9}_1-yGd_y]0_2+x showed positive thermal expansion between 298 Kand 1273 K.