The helium and hydrogen gas permeabilities through proton exchange membranes, poly(vinylidene)-graft-polystyrene sulfonic acid membranes (PVDF-g-PSSA), were studied using a mass spectrometric leak detector. The membranes were prepared by electron beam irradiation followed by grafting with styrene and subsequent sulfonation. In order to separately evaluate the effect of grafting and sulfonation the unsulfonated membranes (PVDF-g-PS) were also studied. The permeability was found to increase with increasing graft content of the unsulfonated membranes. The PS grafts are likely to phase separate from the PVDF matrix during grafting because of the incompatibility of the polymers. Hydrogen and helium permeabilities of PS are higher than of PVDF. Thus the increase in gas permeabilities is most likely due to gas permeation through phase separated polystyrene domains. The gas permeabilities of the dried PVDF-g-PSSA membranes were considerably lower than the values for the unsulfonated membranes. The gas permeability of the dried PVDF-g-PSSA membranes decreased around d.o.g, 35% which is the graft penetration limit. Similarly the ion exchange capacity of the membranes increased at this point. In the water saturated state the permeabilities increased with increasing water content of the membrane. The gas permeability of the water saturated PVDF-g-PSSA membranes reached a plateau value at the graft penetration level. The gas permeation in the water saturated state is mainly governed by the diffusion of the gas in water. The highest permeabilities for PVDF-g-PSSA membranes were of the same order of magnitude as for the commercial Nafion(R) 117 membrane. These permeabilities were measured for membranes with d.o.g. similar to 70%. Thus the PVDF-g-PSSA membranes are good candidates for the use as electrolytes in electrochemical cells.