Uranium–europium mixed oxides (U_1-yEu_y)O_2-x (y=0.2–0.8) were prepared by the citrate gel combustion technique and characterized by X-ray diffraction (XRD). Single phase fluorite structure was observed in those solid solutions with y≤0.6. The solid solutions with y>0.6 were found to be biphasic, with the second phase being cubic Eu₂O₃. Heat capacity and enthalpy increment measurements were carried out by using differential scanning calorimeter (DSC) and drop calorimeter in the temperature range 298–800K and 800–1800K, respectively. The $C_{p,m}$ values at 298 K for (U_1-yEu_y)O_2-x (y=0.2, 0.4, 0.6) are 64.8, 64.6, and 63.5 J·K^-1·mol^-1, respectively. An anomalous increase was observed in the heat capacity in all of the solid solutions with the onset temperature around 950 K. This could be attributed to the contribution from Frenkel pair oxygen defects. From the excess heat capacity data, the enthalpy for the formation of these defects was computed and found to be in the range of 2.10±0.02 eV.