Densities of the negative and positive ions of SF6in a 110‐liter Pyrex container were measured using a Gerdien ion counter. The current‐voltage characteristics were obtained for gas flows ranging from 40 to 200 cm3 s−1. A quantitative model of the counter was developed to interpret the measured data. The model includes appropriate representation of the developing gas flow and the electric field distribution near the ends of the accelerating electrode. The best‐fit value for the negative‐ion mobility was &mgr;n=0.38 cm2 V−1 s−1, as derived from a comparison of the calculated and the measured characteristics. The mobility of the positive ions was found to equal that of the negative ions within the limits of dispersion of experimental data. The negative‐ion density at a pressure of 760 Torr evaluated at the counter entry plane was 3000 cm−3; the value of the positive‐ion density was practically identical.