The self-assembly behavior of a cationic surfactant (dodecyltrimethylammonium) with DNA as counterion in mixtures of water and decanol was investigated. The phase diagram was established and the different regions of the phase diagram characterized with respect to microstructure by H-2 NMR (D2O), small-angle X-ray scattering (SAXS), and other techniques. The DNA-cationic surfactant aggregate is virtually insoluble in water but shows a high solubility in decanol; in decanol, highly viscous solutions/rubberlike solid material is found. In the presence of both solvents, there is a formation of liquid crystalline phases. Both lamellar and reversed hexagonal phases are found, the former favored by higher water contents, the latter by higher concentrations of decanol. The liquid crystalline phases are in equilibrium, via two-phase and three-phase regions, with the essentially pure components.