  We report experimental data of the mobility of O\(_2^-\) ions in argon gas as
a function of the density in the temperature range between \(180\,\mbox{K}\le
T\le 300\,\mbox{K}\). At the intermediate, though fairly large, densities of
the experiment both the kinetic theory and the hydrodynamic theory fail at
describing the experimental data. By contrast, the free volume model,
originally developed to describe electron and ion mobility in superfluid
helium, gives a satisfactory agreement with the data as previously obtained for
the O\(_2^-\) ion mobility in both dense neon and helium gases.
