The thio-Wittig rearrangement of deprotonated allyl methyl sulfide has been examined in the gas phase with a variable temperature flowing afterglow-triple quadrupole device. Collision-induced dissociation studies of a series of thiolate anions (RS-) reveal that methyl deprotonation leads to 3-butene-1-thiolate (2a), the [2,3]-Wittig product, while 1-thiomethylallyl anion (1a) isomerizes to 1-butenyl thiolate (4a), the [1,4]-Wittig product, at elevated temperatures. Activation energies for these processes have been estimated using the Arrhenius equation and are compared to high-level (G2) calculations for the homolytic and heterolytic bond dissociation energies. Stepwise and concerted [1,4] pathways are found to have similar energy requirements, which accounts for some of the mechanistic controversy regarding these transformations in solution. The observed selectivity, [1,2] vs [1,4], is most easily accommodated by a concerted process but can be explained in terms of a stepwise mechanism by considering the spin density and charge location in a radical anion intermediate (6a). Frontier molecular orbital theory, however, leads to the wrong prediction. The [2,3]-Wittig rearrangement appears to proceed via a concerted pathway in the gas phase as has been invoked in the liquid phase. Heats of formation for acrolein (ΔH°(f298) = -15.6 kcal mol-1, thioacrolein (ΔH°(f298) = 37.9 kcal mol-1), and their radical anions (ΔH±(f298) = -1.73 kcal mol-1 and ΔH°(f298) = 16.3 kcal mol-1, respectively) are also provided.