The total ionization produced by ions stopped in nitrogen gas has been measured in the energy range 25-50 keV. The measurements were made using an ion accelerator for producing the incident particles and a cylindrical ionization chamber with a differentially pumped open window. The following ions were studied: H+, He+, C+, N+, O+, and Ar+. The average energy loss per ion pair, W, ranges from 34.6 eV for 25-keV protons to 115.2 eV for 25-keV argon ions. A calculation indicates that a large part of the variation of W with initial ion energy and atomic species may be attributed to the variation of the energy ultimately going into the kinetic energy of the atoms of the gas.