Mass-analysed ion kinetic energy (MIKE) spectra record the translational energy of all the daughter ions formed from a single parent ion in fragmentation reactions in the second field-free region of a reversed geometry double-focusing mass spectrometer. From the individual peak shapes in such spectra, the distribution of kinetic energy releases, n(T), of each reaction can be determined. To glean the information from an experimental peak shape, it is first necessary to determine the exact peak shape that arises when only a discrete energy is released. Previous methods employed to calculate the expected peak shape for a discrete energy release are critically discussed and a new approach is described. The present method is based on the assumptions that the aberrations of a cylindrical electrostatic analyser are not significant compared to its energy dispersion and that the parention beam has a very small angular divergence. An analytical and a computer-based numerical solution have been developed, and they both predict that the peak shape for a single energy release is almost rectangular. In addition, the method has been extended to show how this shape changes when some of the ions are prevented from reaching the detector owing to the finite length of the collector slit. Again, both methods give identical forms for the resultant peak shape. The centre of the original rectangle becomes dished and the depth of the dishing increases in a predictable manner as either the slit length is reduced or the energy release is increased.