Mass identified measurements of the time dependence of the density of atomic argon ions in decaying plasmas are reported. The measurements yield a mobility &mgr;0(Ar+)=1.55 cm2V−1·sec−1for Ar+in argon at a gas temperature of 300°K, and for the rate of conversion of Ar+into Ar2+, a value of 1.9×10−31cm6sec−1.