Current efficiency, amt. and direction of H2O transfer through membrane, and electrolyzer voltage were studied in the electrolysis of aq. 50, 100 or 200 g Na2SO4/dm(3) at 5.0, 7.5 or 10.0 A/dm(2) with DuPont's Nafion 427 used as a cation exchange resin membrane in relation to resulting NaOH concn. (10-290 g/dm(3)) and Na2SO4/H2SO4 conversion in the anolyte. The procedure was found to be advisable only (a) when low Na2SO4 conversion are acceptable (to achieve high current efficiency, low H2SO4 concn. in anolyte, and to leave high unconverted Na2SO4 concn.) and (b) at low initial Na2SO4 concns. (<100 g/dm(3)) to achieve conversion as high as 90%.