The combination of cationic lipids with cationic peptides and DNA vectors can produce synergistic effects in gene delivery to eukaryotic cells. Binary complexes of cationic lipids with DNA are well-studied whereas little information is available about the structure of the ternary lipid/peptide/DNA (LPD) complexes and mechanisms defining DNA protection and delivery. Here we use synchrotron small angle X-ray scattering and dynamic light scattering zeta-potential measurements to determine structure and the net charge of supramolecular aggregates of complexes in mixtures of plasmid DNA, cationic liposomes formed from DOTAP, plus a linear cationic ε-oligolysine with the pendant α-amino acids Leu-Tyr-Arg (LYR), ε-(LYR)K10. These ternary complexes display multilamellar structures with relatively constant separation between DOTAP bilayers, accommodating a hydrated monolayer of parallel DNA rods. The DNA–DNA distance in the complexes varies as a function of the net positive to negative (lipid + peptide)/DNA charge ratio. An explanation for the observed dependence of DNA–DNA distance on charge ratio was proposed based on general polyelectrolyte properties of non-stoichiometric polycation–DNA mixtures.