Results are reported for oxygen and air at 1–10 mm Hg pressure irradiated by 100-kvp X rays at about 4300 roentgens/hour. Measurement of both terms of the complex conductivity of the gas at 50 Mc/s by a bridge technique, with modulation of the X-ray source at 120 c/s, gives data on the collision frequency v, the electron density n, and the dependence of collision frequency on the electron energy u ev. The limit of sensitivity is about one electron per cc. For dry air the data agree well with recent drift tube results by Pack and Phelps represented by v(u) = 1.38 × 10−7N(air) u sec−1, where N(air) is the air density in molecules/cc. For O2 fair agreement with v(u) = 7 × 10−8N(O2)u sec−1 is observed, but the power of u may be greater than unity. The electron densities are consistent with three-body attachment of the electrons to neutral O2 molecule with the coefficient 2.8 × 10−30 cm6/sec reported by Chanin et al. for thermal electrons.