The absolute ratio of fission densities due to thermal- and epicadmium-neutron fluxes has been measured by solid-state track detectors. A systematic deviation from this absolute ratio is observed when measuring gamma activities of fission products. From a careful analysis of the gamma spectra with a Ge(Li) detector, it was concluded that this systematic deviation is due to important changes in the mass distribution of fission products produced by epicadmium neutrons, with respect to the well-known mass distribution for thermal neutrons.