The properties of the liposome membrane as a reaction field were investigated by focusing on the Horner–Wadsworth–Emmons reaction as a case study. Use of the liposomes existing in the gel phase resulted in the enhanced activity of the substrates and furnished the products with same E/Z stereoselectivity as in the liposome-free system. The membrane environment in the gel phase most likely assisted the formation of adducts that induced selective generation of the E-isomer. The possible role of liposomes is to assist the proton removal from the reactant, rather than providing the basic interfacial environment.