Thorium and uranium dioxides form a complete series of solid solutions. The formation from the individual oxides can be performed only by extensive co-milling and reaction at high temperatures. in this study, we prepared 80% ThO2-20% UO2 via coprecipitation of the mixed oxalates from nitrate solutions using an excess of oxalic acid. The parameters which may effect the powder properties were investigated, such as oxalic acid and thorium concentrations, temperature and type of agitation. The specific surface of the oxalates and their derived oxides were measured. Among the precipitation parameters, temperature of 10 degrees C and mechanical stirring resulted in the highest surface area. The coprecipitated Th,U(IV) oxalate powders were decomposed at 350 degrees C, calcinated at 900 degrees C each for 24 h in air and were reduced during 2 h at 650 degrees C in H-2 atmosphere. X-ray diffraction resulted in a single-phase mixed oxide solid solution. The green densities of pellets made from these mixed oxides varied between 36 and 38% TD. Pellets were sintered in flowing CO2 at 1100 degrees C for 30 min and achieved 81% TD. (C) 1997 Elsevier Science B.V.