AbstractPseudo‐first‐order rate constants have been measured for the acid‐catalyzed hydrolysis of a series ofN,N‐dimethyl‐ andN‐methyl‐N‐phenylbenzenesulfonamides in 70% (v/v) CF3CO2H/H2O at 99°C. Analysis of the effect of alkyl substituents in the benzene ring shows a remarkable rate enhancement byortho‐alkyl groups, in contrast with a rate retardation of similar substituents in the corresponding benzamides. The rate enhancement in the sulfonamides is attributed to relief of initial‐state strain in the transition state for hydrolysis. When the sulfonamide moiety is contained in a five‐membered ring fused to a phenyl ring, the rate of hydrolysis is markedly reduced by anor