Ab initio molecular orbital calculations have been performed to study the gas-phase hydrolysis and methanolysis of methanesulfonyl chloride. The overall reaction occurs via a concerted S(N)2 mechanism with a trigonalbipyramidal transition state, and the transition structure is looser when the reaction is catalyzed by additional solvent molecules. The reactivity in a mixed solvent agrees well with the gas-phase proton affinity of the hydrogen-bonded solvent complex, and the methanol-catalyzed methanolysis reaction in the gas phase is the fastest among the reactions discussed. The catalytic role of such solvent molecules appears to be bifunctional as in the hydration of a carbonyl group, but general-base catalysis is more important than general-acid catalysis. The MP2/6-31G* activation energy is reduced considerably by two catalytic water molecules.