The poly(vinylidene fluoride), PVDF, membranes that have been radiation grafted with polystyrene, PS, producing PVDF-g-PS membranes, are intermediates in the synthesis of PVDF-g-polystyrenesulfonic acid (PSSA) polymer electrolyte membranes. Solid-state NMR measurements were carried out to study the structure of the PVDF-g-PS membranes. The previous investigations by thermal methods and by X-ray scattering showed phase separation of the two polymeric components. The extent of phase separation and the mobility of the components in the membranes were investigated by NMR relaxation time measurements and by 2-D wide-line separation (WISE) experiments. It was shown that the amorphous PVDF and PS are phase separated into domains larger than several nanometers. The amorphous PVDF component also retained higher mobility than the PS. Information on the structure of the intermediate product can be used when interpreting the properties of the final polymer electrolyte membrane.