The mobility of potassium ions in nitrogen and neon has been measured by an electrical shutter method of the type developed by Bradbury and Nielsen for measuring electronic drift velocities. The values obtained for the mobility μ₀, 2.54 and 7.42 cm² volt^-1 sec^-1 for nitrogen and neon respectively, are in good agreement with those listed by Tyndall. The variation of the mobility with E/p₀ is shown to be in qualitative agreement with theoretical considerations while the disagreement between the present results and those of Mitchell and Ridler is discussed.