Modifications of an electron impact ion source for a 60° mass spectrometer are described. The changes consist of (1) an almost completely enclosed ionization region which greatly reduces the contribution of ions from thermal decomposition products on the filament and reduces the contribution of background peaks and (2) a built‐in aligned and stronger magnetic field which eliminates source magnet adjustment, collimates the electron beam better and permits the use of a higher repeller voltage thereby increasing the collection efficiency for ions with initial kinetic energy. The performance of the source is described.