We have measured the mobility of ions in air. The method of measuring the mobility is a time‐of‐flight method. The time of flight of the ions is measured between two grids G1and G2, as in Bradbury's method. We have improved the resolution of the apparatus, using a rectangular cyclic voltage, and we have eliminated harmonic peaks. Results with radioactive ions of ThC″ and ions created in air by the &bgr; rays of a tritium source are discussed.