The W value, the mean energy required to form an ion pair, was measured for monoenergetic positive H, He, C, N, O, and Ar ions in the energy range from 1 to 50 keV in air, N2, CH4, CO2, and a tissue-equivalent gas (Rossi-Failla mixture). The W values of ions increase, in general, with decreasing energy; only those of H ions show a minimum at an energy of about 25 keV for some gases. The estimated standard deviation of the measurements is about 2% in most cases.