  A new analysis and new Molecular Dynamics (MD) simulations of the
measurements of the mobility $\mu_{i}$ of O$_{2}^{-} $ ions in dense
supercritical Ar gas are reported. $\mu_{i}$ shows a marked dependence on the
distance from the critical temperature $T_{c}.$ A mobility defect appears as a
function of the gas density and its maximum value occurs below the critical
density. The locus of points of maximum mobility defect in the $P-T$ plane
appears on the extrapolation of the coexistence curve into the single-phase
region. MD simulations quantitatively reproduce the mobility defect near
$T_{c}.$
