Gaseous ammonia at pressures in the range 200 to 1200 Torr was subjected to continuous irradiation by 40 kVp X-rays and the mobilities of the positive and negative ions (μ+ and μ−, respectively) were determined by a pulsed-drift method. The averages of μ+ and μ− agreed well with results obtained by another method. The magnitude of μ+ varied from 3.2 ± 0.2 cm2 V−1 s−1 at 200 Torr to 0.53 ± 0.04 cm2 V−1 s−1 at 1200 Torr. While μ− converged to the same value at 1200 Torr, it was slightly higher, 3.3 ± 0.2 cm2 V−1 s−1, at 200 Torr. The pressure dependence and actual magnitudes of u+ were found to be in good agreement with theoretical predictions based on ion–molecule collision rate theory for NH4+•xNH3 clustered ions of known x.