The present invention provides a process for producing a fluorinated ionomer, comprising copolymerizing, in the presence of a polymerization initiator, (i) a compound represented by Formula (I) (M).sub.1/LOSO.sub.2CFY(CF.sub.2).sub.mO(CFXCF.sub.2O).sub.nCF.dbd.CF.sub- .2 (I) wherein M is H or a metal with a valence of L (L is 1, 2 or 3), X is F, Cl or CF.sub.3, Y is halogen, m is 1 to 5, and n is 0 to 5, (ii) tetrafluoroethylene and (iii) at least one third monomer selected from the group consisting of hexafluoropropylene, chlorotrifluoroethylene, vinylidene fluoride, CF.sub.2.dbd.CFORf (Rf is C.sub.1 to C.sub.5 perfluoroalkyl) and ethylene.The present invention further provides a method for purifying and concentrating a fluorinated ionomer, comprising ultrafiltration or dialysis of a fluorinated ionomer solution or dispersion, and a method for forming a film of the fluorinated ionomer.