The study of hydrolyses of the diesters of benzene dicarboxylic acids as a consecutive reaction and of the sodium salt of the half ester in mixed solvents clearly indicates that both in the first and in the second stage the reaction is only of the ion-dipole type. Calculations of the dimensions of the reactant species show that the most reasonable values are given by the Laidler approach while the Ingold calculations probably give the dimensions of the solvated transition state.