Ionizing potentials for helium and fifteen compound inorganic gases were measured by accelerating photo-electrons through a gauze into a chamber where the positive ions produced were drawn to a fine Pt electrode, made small so as to eliminate effects due to radiation. The current-voltage curves obtained consist of straight lines except at the breaks. Corrections for initial velocity, etc., were made by obtaining on the same curve both a break for the unknown gas and one for mercury (10.4 volts). The values found are, in volts: helium, 24.5; hydrogen, 15.8; nitrogen, 16.3; oxygen, 12.5, 16.1; $\mathrm{HCl}$, 13.8; $\mathrm{HBr}$, 13.2; $\mathrm{HI}$, 12.8; water, 13.2; ${\mathrm{NH}}_{3}$, 11.1; ${\mathrm{Cl}}_{2}$, 13.2; ${\mathrm{Br}}_{2}$, 12.8; ${I}_{2}$, 10.0; $\mathrm{NO}$, 9.4; ${\mathrm{CO}}_{2}$, 14.3; $\mathrm{CO}$, 14.1, 15.6; ${H}_{2}S$, 10.4. Comparison is made with results of other observers. Theoretical interpretation. An attempt is made to test the thermochemical method of estimating ionizing potentials in those cases (${\mathrm{H}}_{2}$S, N${\mathrm{H}}_{3}$, ${\mathrm{H}}_{2}$O) where data are available. No process of ionization involving molecular dissociation seems consistent with the experimental results. This and other evidence suggests that in those cases (HCl, HBr, HI) in which the thermochemical calculations yield accurate results, the agreement may be fortuitous, and that the lowest ionization potentials in each case corresponds to ionization without dissociation.