The radical terpolymerization of 8-bromo-1H,1H,2H-perfluorooet-1-ene with vinylidene fluoride (VDF) and perfluoro(4-methyl-3,6-dioxaoct-7-ene) sulfonyl fluoride is presented. Changing the feed compositions of these three fluorinated comonomers enabled us to obtain different random-type poly[vinylidene fluoride-ter-perfluoro(4-methyl-3,6-dioxaoct-7-ene) sulfonyl fluoride-ter-8-bromo-1H,1H,2H-perfluorooct-1-ene] terpolymers containing various sulfonyl fluoride and brominated side groups. Yields higher than 70% were reached in all cases. The hydrolysis of the sulfonyl fluoride group into the -SO3Li function in the presence of lithium carbonate was quantitatively achieved without the content of VDF being affected, and so dehydrofluorination of the VDF base unit was avoided. These original terpolymers were then crosslinked via dangling bromine atoms in the presence of a peroxide/triallyl isocyanurate system, which produced films insoluble inorganic solvents such as acetone and dimethylformamide (which totally dissolved uncured terpolymers). The acidification Of -SO3Li into the -SO3H function enabled protonic membranes to be obtained. The thermal stabilities of the crosslinked materials were higher than those of the uncured terpolymers, and their electrochemical performances were investigated. According to the contents of the sulfonic acid side functions, the ion-exchange capacities ranged from 0.6 to 1.5 mequiv of H+/g, whereas the water uptake and conductivities ranged from 5-26% (+/- 11%) and from 0.5 to 6.0 mS/cm, respectively. (c) 2006 Wiley Periodicals, Inc.