Lewatit MDS resins from LANXESS significantly increase the efficiency of brine treatment in chlor-alkali electrolysis using the membrane method. The difference between the new resin types and previous versions is bead diameter, which is only 390µm. MDS types are characterized by improved kinetics, a higher degree of regeneration and a significantly increased total and operating capacity, especially for alkaline earth cations. They have optimal mechanical and osmotic stability. The resins provide longer membrane and resin lifetimes; lower operating costs due to lower regenerant requirements and wastewater reduction; longer cycle times in operation and higher throughput in brine preparation.