A phase‐sensitive pulse counting method, which shows considerable promise as a signal processing technique for modulated beam mass spectrometry, is presented. This method allows near‐simultaneous monitoring of mass peaks of interest by fast mass spectrometer scanning, while retaining the advantages of phase‐sensitive detection. The electronic system and the molecular beam apparatus are described, and mass spectrometric results for supersonic water vapor beams, obtained with the system, are presented and discussed.