The formation of monolayers and bilayers of dioleoyl-L-alpha-phosphatidic acid from solutions of small unilamellar vesicles onto polyethylenimine-treated substrates was investigated by means of small angle X-ray scattering and X-ray photoelectron spectroscopy. The formation of monolayers could be verified after an adsorption time of 5 min, on removal of the substrates from the vesicle solution and washing with water. No adsorption of dioleoyl-L-alpha-phosphatidic acid takes place when the substrates used have not been pretreated with polyethylenimine. This suggests that the interaction of the charges of polyethylenimine with the phosphatidic acid headgroups is the driving force for the adsorption. Further experiments show that immersion of the supported bilayers in polyethylenimine solution again leads to dried films with layer thicknesses of approximately twice the values of the supported monolayers. From this observation the formation of supported bilayers of lipid/polyelectrolyte complexes is derived.