A process for the manufacture of a porous diaphragm of a synthetic material, especially polytetrafluoroethylene, which comprises forming a sheet of the synthetic material in admixture with a solid particulate additive (eg starch) to be removed therefrom, introducing said sheet into an electrolytic cell, filling the cell with the working electrolyte and removing the solid particulate in situ in the cell by electrolysing the said electrolyte. The porous diaphragms are especially suitable for use in diaphragm cells for the production of chlorine wherein the working electrolyte is sodium chloride brine.