First order rate constants for the solvolysis of a series of 4-X-benzenesulfonyl chlorides (where X = MeO, Me, Br, and NO2) were measured by the conductance method in H2O over the approximate temperature range of 0 to 25°. Pseudothermodynamic parameters, ΔG≠, ΔH≠ΔS≠, and ΔCp≠were derived and the high accuracy of the rate constants enabled an estimation of dΔCp≠/dT.A trigonal bipyramidal transition state is favored for the nucleophilic attack which is probably by an SN2 type mechanism. A novel explanation for the effect of 4-methoxy on ΔCp≠involves a temperature dependent water–solute interaction, which has general applicability to the behavior of 4-methoxy substituents.