Solid state reactions of UO₂ and ZrO₂ in mild oxidizing condition followed by reduction at 1673 K showed enhanced solubility up to 35 mol% of zirconium in UO₂ forming cubic fluorite type Zr_yU_1-yO₂ solid solution. The lattice parameters and O/M (M = U + Zr) ratios of the solid solutions, Zr_yU_1-yO_2+x, prepared in different gas streams were investigated. The lattice parameters of these solid solutions were expressed as a linear equation of x and y: a₀ (nm) = 0.54704 - 0.021x – 0.030y. The oxidation of these solid solutions for 0.1 6 y 6 0.2 resulted in cubic phase MO_2+x up to700 K and single orthorhombic zirconium substituted a-U₃O₈ phase at 1000 K. The kinetics of oxidation of Zr_yU_1-yO₂ in air for y = 0–0.35 were also studied using thermogravimetry. The specific heat capacities of ZryU_1-yO₂ (y = 0–0.35) were measured using heat flux differential scanning calorimetry in the temperature range of 334–860 K.