Abstract The results of radio-chemical investigations of the fission of uranium, thorium, and bismuth nuclei by 480-MeV protons have been analysed. An interpolation method was used to obtain a full description of the fission products . It was observed that there was a predominant formation (58–64 per cent) of isotopes having a neutron excess, and it is shown that isotopes of maximum yield are essentially amongst these nuclei. Symmetrical fission is most probable in the case of bismuth. For uranium and thorium the fission cross-section is 55 pet cent and 60 pet cent respectively of the geometrical cross-section, for bismuth it is 5 pet cent. The charge distribution of the fission fragments produced by high-energy protons is independent of the mass number of the fission fragments and the atomic number of the fissioning nuclei. The results support a mixed barrier-emissive mechanism for the fission of uranium and thorium nuclei.