Copolymerization, phase behavior, and structural properties of the ternary lyotropic mesophase system formed by the surfactant monomer (2-methaeryloyl-oxyethyl)dodecyldimethylammonium bromide (1), styrene, and water are described. Lyotropic mesophases occur if the total monomer concentration is higher than 55 wt % and 1 is the major component. At low styrene content (1/styrene molar ratio of 10: 1 or 5: 1), hexagonal (H-alpha) and cubic (Q(alpha)) phases are formed, while at high styrene content (1/styrene ratio of 3:1) a lamellar phase exists. The lyotropic phases can be polymerized upon gamma-irradiation. At low conversion, a styrene-rich copolymer is formed, whereas at high conversion, a surfactant-rich copolymer is obtained. Homopolymers are not detected. Small-angle X-ray scattering measurements and polarizing microscopy indicate that the structure of the H-alpha-phase is preserved upon the polymerization. The L-alpha-phase undergoes a transition into the Halpha-hase. Independent from the comonomer ratio, nanostructured gels with hydrophilic and hydrophobic compartments are formed. Structure models of the copolymerization process are presented.