[en] The oxalate complexing of Th(IV), Np(IV) and Pu(IV), in aqueous solutions, was studied by the dinonylnaphtalenesulphonic acid (DNNS) extraction method in 1M perchloric acid at 25 deg C and the stability constants obtained are reported. The study of the oxalate complexing of tetravalent actinide ions is of interest as their oxides are usually prepared via their oxalates. The stability constant values for the oxalate complexing of these ions indicate the order Th(IV) >Np(IV)>Pu(IV). Th-234 tracer was separated from an old stock of nuclear grade natural uranium. Th-232, as nuclear-pure thorium nitrate, was obtained from M/s Indian Rare Earths. Np-237 (AERE-UK) was purified by the thenoyltrifluoroacetone (TTA) extraction method. Np-238 was produced by neutron irradiation of Np-237 and purified by TTA extraction from the fission products. Pu-238, which was present in a consignment of Np-237, was separated and purified. (T.I.)