Copolymerization, phase behavior, and structural properties of the ternary lyotropic mesophase system formed by the surfactant monomer (2-methacryloyl-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 (HR) and cubic (QR) 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 γ-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 HR-phase is preserved upon the polymerization. The LR-phase undergoes a transition into the HR-phase. Independent from the comonomer ratio, nanostructured gels with hydrophilic and hydrophobic compartments are formed. Structure models of the copolymerization process are presented.