The experimental delayed-neutron spectra of 25 precursors in fission have been described in terms of a series of fine structure
peaks superimposed on a gross structure consisting of 1–3 components corresponding to different wave-numbers. These precursors
are responsible for 85–87% of the delayed-neutron effect in nuclear fuel. By including also some precursors whose spectra
have been deduced by an extrapolation procedure the effective delayed-neutron energy distribution in nuclear fuel can be accurately
calculated. As a byproduct the applicability of some recent mass formulas in the region of very neutron-rich nuclei has been
tested. Systematic deviations have been found, which indicate the need of further investigations.