The complexation of bilayer-forming lipids, dihexadecyl phosphate (DHP) and soja-lecithin, with a cationic polyelectrolyte (PDADMAC) results in stable, highly ordered mesomorphous materials. For the DHP complex, a close to perfect L(beta)’ morphology is found with SAXS. The increase of the phase transition temperature from T-c = 66 degrees C for the free DHP membranes toward T-c = 84.4 degrees C for the polyelectrolyte-lipid complex reflects the increased membrane stability due to polymeric counterions. Due to an extremely high glass transition of the ionic interlayers, the DHP complex is a nonplastic and brittle solid. The lecithin complex with its mixture of head groups and tail lengths exhibits, opposite to its clean synthetic counterpart, a quite unconventional phase structure where the stack of lamellar bilayers undulates with very high amplitudes. A similar undulated structure has recently been postulated for the natural lecithin membrane(1-3) and is obviously necessary for some biophysical membrane functions. In addition, a remarkably improved mechanical behavior of the lecithin-complex film paralleled by the depression of the glass transition of the ionic layers to T-g = 10 degrees C is observed which allows large amplitude deformation, mechanical orientation as well as thermomechanical processing of the complex. Such systems can be understood as "plastic membranes", are comparably cheap, and might be interesting as materials themselves. The observation of undulations as well as the unexpected good mechanical properties of the polymer-lecithin complex underlines that such three-dimensional materials in the bulk may act as model systems for the special properties of lecithin mesophases which arise from an appropriate mixture of tails and head groups, optimized by evolution processes.