Octadecyltrichlorosilane-treated H-ZSM-5, abbreviated as H-ZSM-5-C18, resulted in an effective catalyst for the hydrolysis of water-insoluble esters in toluene-water solvent system. These floated at the interface of the two liquids. The H-ZSM-5-C18 catalyst showed ca. 60 times the activity of H-ZSM-5 for the hydrolysis of dodecyl acetate. Acid strength of H-ZSM-5 in water (-5.6 < Ho less-than-or-equal-to -3.0) was maintained even after the treatment with octadecyltrichlorosilane. The treatment renders H-ZSM-5 lipophylicity and provides a quasi-lipid phase on the surface. The shape-selective properties of H-ZSM-5-C18 catalyst were observed for the hydrolysis of aliphatic esters with straight chains.