The oxidation states of tin, antimony and tellurium isotopes formed predominently (>91%) by fission were investigated in aqueous solutions of 233U irradiated in a reactor. Results were corrected for the thermal reactions with the bulk radiolysis products using tracers for each element. In 0.4 M H 2SO 4 solutions containing 4.3 mM 233UO 2 2+, 1.5 mM Te(IV) and 1.5 mM Te(VI), the fraction of Te(IV) is 98 ± 2% in the absence of oxygen and 92 ± 3% in the presence of oxygen. In frozen solutions of the same composition, the fraction of Te(IV) is 83 ± 2% in the absence of oxygen and 81 ± 3% in the presence of oxygen. Similar differences are also found in the case of the antimony isotopes in 0.4 M H 2SO 4 solutions and the tin isotopes in 1 M HCl solutions. For the interpretation of the differences, the reducing action of the species diffusing from the track of the fission fragments are considered.