[reaction: see text] Kinetic data indicate that the hydrolysis of S-2,4-dinitrophenyl 4'-hydroxythiobenzoate in mild alkaline solutions (pH 8-11) most likely follows a dissociative, E1cB pathway, through a p-oxoketene intermediate, whereas at higher pH values an associative mechanism carries the reaction flux. Free linear energy relationships obtained from a kinetic study on the alkaline hydrolyses of substituted S-aryl 4'-hydroxythiobenzoates seem to suggest that the associative pathway is a concerted, one-step process, rather than the classical mechanism via a tetrahedral intermediate.