The kinetic solvent isotope effect, was measured for 4-X-benzenesulfonyl chlorides (where X = MeO, Me, H, Br, and NO2) and shown to vary systematically, giving linear correlations with Hammett σ values and withratios. These results contrast with the lack of sensitivity in k.s.i.e. to structural changes for displacement from a saturated carbon atom. This behavior is attributed to a greater degree of bond making required to achieve a critical charge on the chloride of the sulfonyl chloride at the transition state.