The collisional broadening of peaks in the spectra arising from translational energy spectroscopy (TES) studies is theoretically investigated. A numerical calculation, based on the TRIO matrix ion-beam transport computer programme, is used to simulate the collision event and its effects on the focussing properties of a number of ‘double-focussing’ instrument designs. The ion-optical models utilised include a commercial mass spectrometer and two novel high-resolution energy spectrometers (TESI and TESII), incorporating quadrupole and hexapole field lenses to focus the beam and reduce image aberrations. For a given design of spectrometer, peak broadening is evaluated in terms of the translational energy change suffered by the ion during collision and the angle through which it is scattered.