Five mixed metal oxalates of lanthanides and uranium of the formula (NH4)Ln(III)U(IV)(C2O4)(4) . 8H(2)O were prepared and characterised by chemical, infrared spectroscopy, X-ray diffraction and thermal methods. The X-ray powder diffraction data of all the compounds were indexed on an orthorhombic unit cell. Thermal decomposition of the compounds proceeded with the loss of water molecules below 475 K followed by decomposition in the temperature range of 625 to 655 K to give Ln(2)O(2)CO(3) and UO2+x as intermediate products. The intermediate products reacted above 975 K to form solid solutions of mixed oxides having a fluorite structure. Activation energy values were derived for the dehydration of lanthanum and terbium compounds. Lattice parameters of the fluorite phases of the mixed oxides formed by heating the compounds in air, argon and N-2/H-2 have been determined.