The aim of the present study is the preparation of ThO2-UO3 gel microspheres up to 40% UO3, suitable as press feed for producing homogeneous and sinterable ThO2 and thorium based (Th,U)O-2 pellet type fuels using sol-gel process. Without reducing the uranium oxidation state to (TV) and without using any complexing agent, a simple external gelation process was studied taking full advantage of the gelation features of thorium. The source sols used for the processes were prepared by the ammonia addition method where starting thorium and uranium nitrate solutions were heated and partially neutralized by aqueous ammonia under pH control at different pH set and neutralization mode for each uranium mole ratio. Crackfree microspheres suitable for gelation were obtained using a hexone-(10%) CCl4 mixture as the drop formation medium and ammonia as the gelling agent. The dimensions, sphericity, bulk densities and specific surface area of microspheres were determined. The microspheres were compacted and then the pellets were sintered in a 75% Ar-25% H-2 atmosphere at 1100 degrees C for 150 min. (C) 1998 Elsevier Science B.V. All rights reserved.