Hydroxide ion catalysed hydrolysis of a series of diaryl phosphates with varying leav­ing groups, has been studied in water at 390C and an ionic strength of 1.0. Mechanism involves a bimo­locular nucleophilic attack of the hydroxide ion concerted with the departure of the leaving group. A linear free energy relationship has been determined with a slope of -0.68 for a logarithmic plot of the second-order rate cosntants for the hydrolysis, against the pK of the leaving groups.