An experimental method of measuring epithermal-neutron spectra by use of resonance activation detectors is presented. Measurements of the change in cadmium ratio with varying dilutions of the same isotope are related to the self-screening properties of selected neutron resonances. The neutron spectra obtained in this way are compared with time-of-flight measurements. A wide range of 235U- or 233U-fueled, BeO-moderated substitution lattices were studied. Marked deviation from the 1/E distribution of the epithermal spectrum was observed for lattices in which the fuel-to-moderator ratio exceeded 1:2000.