In continuation of previous work, we present a new promising DNA carrier, OO4, a highly effective peptide mimicking lysine-based cationic lipid. The structural characteristics of the polynucleotide carrier system OO4 mixed with the commonly used co-lipid DOPE and the saturated phospholipid DPPE have been studied in two-dimensional and three-dimensional model systems to understand their influence on the physical chemical properties. The phase behavior of pure OO4 and its mixtures with DOPE and DPPE was studied at the air water interface using a Langmuir film balance combined with infrared reflection-absorption spectroscopy. In bulk, the self-assembling structures in the presence and absence of DNA were determined by small-angle and wide-angle X-ray scattering. The amount of adsorbed DNA to cationic lipid bilayers was measured using a quartz crystal microbalance. The choice of the co-lipid has an enormous influence on the structure and capability of binding DNA. DOPE promotes the formation of nonlamellar lipoplexes (cubic and hexagonal structures), whereas DPPE promotes the formation of lamellar lipoplexes. The correlation of the observed structures with the transfection efficiency and serum stability indicates that OO4/DOPE 1:3 lipoplexes with a DNA-containing cubic phase encapsulated in multilamellar structures seem to be most promising.