It is shown that the ion cyclotron resonance absorption line in weakly ionized gases may be interpreted to obtain em ratio, the number, and the collision frequency of the ions. From the observed linewidths, the collision frequencies of several ions are found as a function of pressure and electric field strength. From these data the ion-molecule collision cross section for N+2 and Ar+ in their parent gases are found to be 125 and 140 Å2, respectively, at high Ep and 185 and 215 Å2, respectively, at low Ep. Observed absorption and dispersion line shapes fit closely the Lorentz line shape derived theoretically with the assumption of constant mean free time. The measurement chamber consists of an ion source and drift region in a strong magnetic field within which a transverse rf electric field may be applied. A narrow band, derivative-type system is used to detect the power absorption as a function of frequency.