Fluorinated ion exchange polymers which have pendant side chains which contain --OCF.sub.2 COOR groups, or which have both pendant side chains containing carboxylic groups and pendant side chains which contain sulfonyl groups, when used in the form of membranes to separate the anode and cathode compartments of an electrolysis cell, permit operation at high current efficiency. Those containing both carboxylic and sulfonyl groups can be made by oxidation of fluorinated polymers which have pendant side chains containing sulfinic groups, or both sulfinic and sulfonyl groups. The fluorinated polymers which have pendant side chains containing sulfinic groups, or both sulfinic and sulfonyl groups, are in turn made from fluorinated polymers which have pendant side chains containing sulfonyl halide groups by reduction with, for example, hydrazine. Those polymers which have only pendant side chains which contain --OCF.sub.2 COOR groups can be made by copolymerization of a mixture of monomers, one of which is a vinyl monomer which contains the indicated carboxylic group. Fluorinated ion exchange polymers which have pendant side chains containing --OCF.sub.2 COOR groups and also pendant side chains which contain sulfonyl groups can also be made by copolymerization of a mixture of monomers, one of which is a vinyl monomer which contains the indicated carboxylic group; and also by treatment of a polymer which contains --OCF.sub.2 CF.sub.2 SO.sub.3 H or salts thereof with a combination of fluorine and oxygen.