Solvolyses of methanesulfonyl chloride in water, $D^2O$, $CH^3OD$, and in aqueous binary mixtures of acetone, eth-anol and methanol are investigated at 25, 35 and $45^{circ}C.$ The Grunwald-Winstein plot of first-order rate con-stants for the solvolytic react ion of methanesulfonyl chloride with YCl (based on 2-adamantyl chloride) shows marked dispersions into three separate lines for three aqueous mixtures with a small m value (m < 0.30), and shows a rate maximum for aqueous alcoholic solvents. Stoichiometric third-order rate constants, kww and kaa were calculated from the observed first-order rate constants and (kaw + kwa) was calculated from the kww and kaa values. The kinetic solvent isotope effects determined in water and methanol are consistent with the proposed mechanism of the general base catalyzed and/or SAN/SN2 reaction mechanism for methanesulfonyl chloride solvolyses based on mass law and stoichiometric solvation effect studies.