An improved diaphragm for electrolysis, said diaphragm being composed of a fibrous base material of asbestos or a mixture of asbestos and fluorine-containing fibers and existing in the interstices of said base material, a water-swellable micaceous mineral represented by the following general formulaW.sub.1/3-1.0 [X.sub.2.5-3.0 (Z.sub.4 O.sub.10)F.sub.2 ]wherein W is Na and/or Li, X is Li and/or Mg, Z is Si and/or Al, O is oxygen and F is fluorine. The diaphragm can be produced by dispersing a fibrous base material of asbestos or a mixture of asbestos and fluorine-containing fibers in an aqueous slurry of the water-swellable micaceous mineral being partly in the form of a sol or in a mixture of it with an aqueous solution of sodium hydroxide and/or sodium chloride, forming the dispersion into a sheet, and drying the sheet.