Abstract—The thermal polymerization of styrene sorbed from the gas-phase into polymer films of polyvinylidene fluoride (PVDF) is carried out at 110°С. By this method, the “matrix‑polystyrene” composites containing up to 70 wt % polystyrene (PS), which serve as precursors of ion-exchange membranes, are synthesized. Sulfonation of grafted PS produces ion-exchange membranes with the exchange capacitance of 1–2.7 mmol/g and the protonic conductivity reaching 20–200 mS/cm when saturated with water at 25°С. The conductivity values indicate that the nonuniformity of PS distribution over film-matrix cross-section usually encountered when monomer sorbed from the gas phase is polymerized does not exert any noticeable effect on the conduction properties of sulfonated composites. The developed method of preparing composites “polymer matrix‑grafted polystyrene” substantially simplifies the synthesis of the precursor of ion-exchange membranes, decreases the necessary amount of reagents, and considerably enhances the safety of synthesis.