The structure and proton conductivity of sulfonated styrene−ethylene copolymers have been studied. Conductivities in excess of 0.1 S/cm are obtained depending upon copolymer composition and sulfonation level. The dependence of conductivity on humidity has been measured and compared with that of Nafion and a partially sulfonated block copolymer. X-ray and neutron scattering studies suggest the presence of a bicontinuous network of hydrophilic and hydrophobic domains in water-swollen samples.