High-fluence proton irradiation experiments with thin actinide targets are part of PSI's programme related to accelerator-based transmutation studies. The aim is to provide experimental data for the validation of theoretical models in nucleon-meson transport codes, with emphasis on the mass yield distribution of spallation and fission products. The paper presents results of an irradiation experiment carried out with 238UO2 and 232ThO2 Based on mass spectrometric measurements, isobaric production cross-sections for fission and spallation products, as well as total fission cross-sections of the target nuclides, are reported. The experimentally deduced total fission cross-sections for both target nuclides are found to be approximately twice as large as values reported in certain earlier experiments, as well as with respect to calculational results obtained using the HETC code and the RAL high-energy fission model