The fission cross-section ratios 239Pu: 235U, 233U: 235U have been measured as a function of neutron energy between 5 keV and 1 MeV with an accuracy between 1.5 and 3%. Fission events were detected in 4 π-geometry by means of argon-filled gas scintillation chambers. The neutron energy has been determined by the time-of-flight technique. The results were compared with the evaluations of Davey. For the ratio 239Pu: 235U there is fairly good agreement between the measured values and the evaluation of Davey, except in the energy region between 200 and 800 keV. There, our values are ∼2 to 4% lower than those of Davey. The 233U: 235U ratio agrees with Davey's curve for neutron energies <60 keV. Above this energy, our results are ∼5% higher than the evaluated curve of Davey.