Synthetic-fiber diaphragms are further improved by incorporating in the diaphragm an effective proportion of a suitable inorganic material such as TiO.sub.2, BaSO.sub.4 or K.sub.2 Ti.sub.8 O.sub.17, which is more hydrophilic than the fluoropolymer forming the diaphragm. This is done either by mixing the inorganic material with the resin before it is made into fiber or by supplying sub-micron-sized particles of the inorganic material, during or even after diaphragm formation. A principal benefit is that this lowers the cell voltage which is required during an initial period (up to about 300 hours) of the operation of a chlor-alkali cell provided with such a diaphragm, making it possible to avoid such drawbacks as suffering an initial period of low production or the necessity of providing external cooling to the cell during such an initial period.