We have achieved the synthesis and isolation, without substantial label scrambling, of 2-(phenylthio)-ethyl-2,2-d2 chloride (2-Cl), a compound useful in modeling reactions of mustard and other highly reactive beta-thioethyl chlorides. 2-Cl undergoes slow label scrambling upon dissolution in hot, dry acetonitrile or acetone. Rates of scrambling of the deuterium label in 2-Cl have been measured by following proton NMR changes. Our present work clearly demonstrates that cyclic sulfonium ions are involved in these scrambling reactions. In anhydrous or aqueous acetone, thiourea traps the sulfonium ion intermediate to give stable, label-scrambled thiouronium salts. In anhydrous acetone, in competition with formation of the thiouronium salts, chloride return occurs giving 2-Cl and its label-scrambled isomer, 2-(phenylthio)ethyl-1,1-d2 Chloride (1-Cl). However, no chloride return is observed in 60% aqueous acetone. Mechanistic implications of these results are presented.