Small-angle and wide-angle X-ray scattering and anomalous small-angle X-ray scattering were used to investigate proton-conducting membranes prepared by radiation-induced styrene grafting and sulfonation of commercial poly(vinylidene fluoride) (PVDF-g-PS) films. The membranes retain the lamellar and highly oriented structure of the original PVDF films even through excessive grafting and sulfonation. The sulfonate groups aggregate in the central part of the amorphous layers, where they form a weakly ordered structure that does not show any preferred orientation. This structure is suggested to be lamellar with alternate metal-sulfonated hydrate and PVDF-g-PS layers. The lamellar period is 15.1 Angstrom.