The lyotropic L-phase of hydrated dipalmitoylphosphatidylcholine has been proposed as a solvent medium for modeling the effects of membranotropic agents upon cell membranes, including interactions between different agents inside the membrane. For quasibinary systems containing bisquaternary ammonium compounds decamethoxinum or aethonium and 2,5-dihydroxybenzoic acid, calorimetry data allowed us to establish the stoichiometry of the intermolecular complexes incorporated in the membrane. The interaction mechanisms have been clarified using additional data obtained with tetramethylammonium and l-menthol, which model the moieties of the decamethoxinum molecule separated by the alkyl chain. Possible implications of the obtained results for pharmacology are discussed.