The total ionization produced by ions stopped in argon gas has been measured in the energy range 8-100 keV. The experimental method employed an ion accelerator and a cylindrical ionization chamber with a differentially pumped open window. The following ions were studied: hydrogen, helium, carbon, nitrogen, oxygen, and argon. The average energy loss per ion pair (W value) was found to be dependent on both the velocity and mass of the incident ions at low ion velocities. The W value, in electron volts per ion pair, ranges from 26.8 for 50-keV protons to 90.9 for 25-keV argon ions.