Shape-selective properties of octadecyltrichlorosilane-treated H-ZSM-5, abbreviated as H-ZSM-5-C-18, have been observed in the hydrolysis of esters having some rings and lactones in toluene-water solvent system. The shape-selectivity for the reaction has been evaluated by the ratio k’(HCl)/k’(H-ZSM-5-C18) of the relative rate constants in comparison with the rate constant of methyl acetate. The selectivity became higher with increase in bulkiness of the substrate. Substrates having the minimum diameter larger than 6.5 Angstrom, significantly larger in size than the pore openings of ZSM-5, could not react in this system.