X-ray diffraction, chemical and thermal studies of [(U_0.2Th_0.8)_1-yLa_y]O_2+x (LUTL) and [(U0.3Th0.7)1-yLay] O2þx (UTL); compounds (where y  0.4) were carried out. These compounds were synthesized by gel combustion method followed by heating in reduced atmospheres at 1673 K. To co-relate lattice parameters with metal and oxygen concentrations, reduced oxides were heated in Ar, CO2 and air atmospheres. Retention of FCC phase was confirmed in all mixed oxides with y  0.4. The cubic lattice parameters could be expressed in a linear equation of x and y as: a (Ǻ) ¼ 5.5709 ¡ 0.187 x þ 0.032 y; [x < 0 and 0  y  0.40] for LUTL and a (Ǻ) ¼ 5.5580 ¡ 0.26 x þ 0.015 y; [x < 0 and 0  y  0.36] for UTL. Oxidation studies and simple ionic model calculations suggested that uranium is predominantly present as a mixture of þ5 and þ 6 states when La/U ratio ~2. Oxidation kinetics of mixed oxides was studied by non-isothermal method using thermogravimetry and was found to be a diffusion controlled reaction. High temperature X-ray diffraction studies of LUTL and UTL mixed oxides showed positive thermal expansion in the temperature range of 298e1273 K and % expansion increases with La concentration.