An electron impact ion source suitable for use with a quadrupole mass filter is described. Ion formation occurs throughout a relatively large volume since no magnetic field collimation of the ionization electron stream is used. A sensitivity of 8 Torr−1was obtained for an ion energy of 50 eV rising to a constant sensitivity of 12 Torr−1for ion energies in excess of 250 eV. Trajectory tracing has been used to give some insight into the efficient operation of the ion source.