The paper describes the development of a crossed beam method suitable for measuring ionization cross sections of unstable gases such as atomic hydrogen.Results obtained in He and H₂ are compared with those of Tate and Smith. The method makes use of a narrow band amplifier associated with a beam chopper and phase sensitive detector, to eliminate the current due to ionization of the background gas. The electric field in the collision region is such that limitation of the beam crossover volume can make the uncertainty in electron energy much smaller than is otherwise possible, without resorting to magnetic collimation or pulsed collection. An analysis of the sources of error due to the collection of spurious currents at signal frequency is given.