Thorium oxalate dihydrate powders were obtained by precipitation from thorium nitrate solution using oxalic acid in the presence of ultrasound and mechanical agitation. Properties measured included particle morphology, particle size and particle size distribution, and specific surface area. The thermal decompositions of the powders were investigated by using thermogravimetry and differential thermal analysis techniques. The result showed that the thermal decomposition stages of the thorium oxalate dihydrate obtained by mechanical and ultrasonic agitation are same. The decomposition reactions of the oxalates precipitated in the presence of ultrasound proceed in a narrower temperature interval, within a shorter period of time. The kinetic parameters for each stage were calculated. (C) 2001 Elsevier Science B.V. All rights reserved.