The interaction of DNA with other DNA and with charged interfaces is of utmost importance for understanding nonviral transfection and DNA diagnostics with optimized chips. This can be studied in detail using Langmuir monolayers of cationic lipids as soft surface of a DNA containing subphase. The positional order of the lipid and the DNA sublattices can be studied by synchrotron X-ray diffraction under continuous variation of parameters such as molecular density or surface pressure. It is shown that DNA binding condenses the membrane surface, and the resulting structure is determined by the interplay of DNA-lipid and DNA-DNA interactions.