A method is described for performing electron‐impact ionization time‐of‐flight mass spectrometry in a molecular beam apparatus. It provides a convenient means for optimizing the performance of pulsed or continuous nozzle sources and can be used in conjunction with laser excitation. Mass spectra are produced either as analog waveforms or in a high repetition rate ion counting mode. The device can also be operated as a fast ionization gauge for time‐resolved detection of pulsed beams.