The mechanism of the alkaline hydrolysis of phosphate and sulfate esters is of great interest. Ab initio quantum mechanical calculations and dielectric continuum methods are used to investigate the effect of the solvent on the associative/dissociative and the in-line/sideways character of the hydrolysis reaction of ethylene sulfate (ES) and ethylene phosphate (EP-), and their acyclic counterparts, dimethyl sulfate (DMS) and dimethyl phosphate (DMP-). The gas-phase reaction coordinates are determined by Hartree-Fock and density functional theory. For ES, the reaction coordinate in solution is determined: for the other three reactions only the transition state in solution is obtained. The alterations in the reaction induced by solvent are interpreted by use of the Hammond and anti-Hammond postulates.