We report a comprehensive study of the lamellar dioleoyl trimethylammonium propane and DNA complexes (DOTAP-DNA) as a function of the lipid/DNA weight ratio rho in order to determine their structure and thermodynamic stability using an energy dispersive X-ray diffraction technique (EDXD). These self-assemblies consist of a periodic multilayer structure with DNA chains adsorbed between cationic membranes. Furthermore, above and below the isoelectric point, the DNA packing density significantly varies as a function of rho. Our results confirm that EDXD is a powerful technique to characterize self-assembled ordered structures of oppositely charged macromolecules, (C) 2002 Elsevier Science B.V. All rights reserved.