Abstract: The present invention relates to a new type of diaphragm for use in electrolytic cells for the production of hydrogen and oxygen, which diaphragm consists essentially of fluorocarbon polymers which have been oxidized to improve its wettability. According to the present invention the improved diaphragm is characterized in that the fluorocarbon polymers are present in the form of monofilaments, that only the surface of the monofilaments is oxidized, and that the monofilaments are woven into a fabric. In a preferred embodiment the monofilaments have a diameter of less than 0.25 mm in the weft and less than 0.15 mm in the warp. The permeability of gas in the dry state is from 50 to 1000 l.dm.sup.-2.min.sup.-1 when the difference in pressure is 200 Pa. Preferably the monofilaments are evenly spaced to effect even size of the pores between the monofilaments.