A modification of the ion source of a time‐of‐flight mass spectrometer is described which permits measurement of the angular spread of the ion beam leaving the ionization region. From these measurements, it is possible to obtain kinetic energy information for a specific, selected ionic species in a mass spectrum. In the modified ion source, ions leave the ionization region through a small, circular aperture, and then enter a region with a uniform accelerating electric field. Before they enter the drift tube for mass analysis, the ions must pass through a second small aperture, which is movable in a direction perpendicular to the flight tube of the mass spectrometer. The instrument has had all magnets removed. Analysis of the ion beam cross‐section profile yields values for excess kinetic energy and the relative abundances of ions which are formed from molecular states excited by electron impact. Typical results are presented.