The fission cross-section ratios of 239Pu to 235U and 233U to 235U have been measured from 0.24 to 24 keV using parallel-plate, back-to-back fission detectors and a slowing down lead spectrometer as a neutron source. Relative fission rates obtained from the experiments were converted to true fission cross-section ratios by detector intercalibration in a thermal-neutron flux. In the case of the Pu/U ratio, these results extend the data to below several keV where measurements have not previously been made and the 233U/235U results verify previous measurements made in this energy region.