Mass‐identified measurements of the time dependence of the density of atomic neon ions in decaying plasmas are reported. The conversion of Ne+intoNe2+occurs by the three‐body reactionNe++2Ne→Ne2++Neat a rate given by the rate constantK=7.7×10−32cm6sec−1in good agreement with the value reported by Smith and Cromey. The measured value of mobility of Ne+ions in neon &mgr;0(Ne+) = 4.0 cm2V−1sec−1at a temperature of 300°K is in agreement with the values obtained by other techniques.