A technique for measuring the speed distribution of molecular beams which uses symmetric modulation and phase‐sensitive detection is described. Corrections for system errors on experiments of this type are developed and verified. In addition to the expected 1/vdependence of the ionizer, the extraction efficiency of the quadrupole mass spectrometer detector is shown to depend on the velocity of the molecules before ionization. The prediction of 5.5% energy enhancement of beams from channel sources operated at pressures well above the free‐molecule flow limit has been verified.