Measurement of the Ionization potential of Gases was accomplished by means of a two electrode tube, by determining the point on the current-voltage curve at which the current increases at a rate faster than that given by Langmuir's equation $i=A{(V+{V}_{0})}^{\frac{3}{2}}.$The effect of a voltage drop along the cathode was eliminated by a commutator arrangement which broke the filament heating circuit while the electron current was being measured.The initial velocity of the electrons, ${V}_{0}$, was determined directly from the currentvoltage curve.Ionization potentials were obtained by measurement as follows: Argon 15.6, nitrogen 15.8, carbon monoxide 15.0, hydrogen 15.1, helium 20.5, mercury vapor 10.1, iodine vapor 8.5. The ionization potential of argon was found to be constant for pressures between 1 and 200 bars.