Abstract The electrolytic reduction process of actinide oxides in a LiCl salt bath at 923 K has been developed for nuclear fuel reprocessing. Since some salt-soluble fission products, such as Cs, Sr and Ba, accumulate in the LiCl salt bath, their effect on UO 2 reduction was investigated. In the experiments, UO 2 specimens were reduced by potential- or current-controlled electrolysis in various LiCl salt baths containing up to 30 mol% of KCl, CsCl, SrCl 2 or BaCl 2. The rate of UO 2 reduction in a LiCl salt bath was considerably decreased by the addition of alkali metal chlorides (KCl and CsCl) and slightly decreased by BaCl 2 addition. SrCl 2 addition had no appreciable effect. It was suggested that the diffusion of O 2− ions from the inside of UO 2 specimens to the bulk salt determined the reduction rate during the electrolysis and that the effect of salt composition was related to the solubility of O 2− ions in the salt bath.