A method of operating a chlor-alkali electrolytic cell comprising a catholyte compartment containing a cathode, an anolyte compartment containing an anode, and a liquid-permeable diaphragm partitioning the catholyte and anolyte compartments, is described. The method comprises adding water-insoluble inorganic particulate material, e.g., clay mineral, and alkali metal polyphosphate, e.g., tetrasodium pyrophosphate, to the anolyte compartment of the electrolytic cell while the cell is operating. The water-insoluble inorganic particulate material and alkali metal polyphosphate may be added to the anolyte compartment in the form of an aqueous slurry.