The reactivity-pH profile for the hydrolysis of 4-aminobenzenesulfonyl chloride 1 has a sigmoid shape with a plateau extending from pH 2 to 11; reactivity of N,N-dimethyaminobenzenesulfonyl chloride 4 is invariant over an even wider pH range (0-14.7). These results, together with the activation data determined at selected pH's for compound 1, are interpreted in light of the occurrence of a reaction mechanism that is dissociative in nature, in which nucleophilic assistance by solvent molecules is given to the amino group of 1 acting as an "internal nucleophile".