An electronegative gas detector has been developed which is capable of measuring one part of SF6in 107parts of air. The operation of the instrument is based on the differences between various electronegative gases in the rates of electron attachment at low electron energies, and on the differences of mobilities of the resulting negative ions in an electric field. The instrument operates at atmospheric pressure (E/p∼1 v/cm‐mm Hg) and uses a modulated ultraviolet light source to produce at a photocathode pulses of photoelectrons which are attached to produce negative ions. The differences in cathode‐anode transit times of the negative ions result in different phase shifts of the observed cathode current. The measured phase shifts serve to indicate the amount of electronegative gas present.