The 240Pu-to-235U fission cross-section ratio has been measured at 55 discrete energies between thresh-old and ∼10 MeV using the 7Li(p,n)7Be and D(d,n)3He reactions as neutron sources. The sample masses were measured by (a) calculated specific activities and low geometry alpha counting, (b) by mass spectrographic isotopic dilution analyses, and (c) by comparing the relative thermal fission rates of one of the 235U samples with 240Pu samples containing ∼9% 239Pu.