Integral values of the capture-to-fission cross-section ratio for 239Pu and 233U have been measured in a soft spectrum fast reactor critical assembly using mass spectrometric techniques to determine the capture rate. Values of 0.363 ± 0.024 and 0.10 ± 0.04 were obtained for 239Pu and 233U, respectively. The value of 0.363 ± 0.024 is in good agreement with the value calculated using the revised Gwin differential cross-section data for 239Pu. The value for 233U is within experimental error in agreement with the value calculated using ENDF/B data.