We have measured the fission cross-section ratios 234U:235U, 236U:235U, and 238U:235U as a function of neutron energy from 0.1 to 30 MeV using ionization fission chambers and the time-of-flight technique at the Lawrence Livermore Laboratory 100-MeV electron linear accelerator. Our experimental procedure required accurate measurements of the isotopic composition of fissionable material within fission chambers containing a mixture of the threshold isotope (i.e., 234U, 236U, or 238U) and 235U. The measured cross-section ratios, averaged over the neutron energy interval 1.75 to 4.00 MeV, are 1.216 ± 0.012, 0.7166 ± 0.0072, and 0.4422 ± 0.0031, respectively.