The content of radicals in the base polymer film before and after grafting is an important issue in the production and use of membranes for electrochemical devices by radiation grafting. In this work a method has been developed for determination of relative radical content in fluoropolymer films using MgO as internal standard. It has been demonstrated that peroxy radicals generated by electron irradiation of ETFE decay rapidly within the first day after the irradiation, probably due to recombination within the amorphous domains. The extent of decay can be reduced by lowering the storage temperature which decreases polymer chain mobility. At room temperature decay of radicals in the crystallites too is indicated, although at a lower rate. In order to obtain reasonable grafting yield storage should therefore be as short or cold as possible. All radicals have been found to disappear in the grafting process, and no significant amount of potentially harmful radicals is thus left in the final membrane. (C) 2008 Elsevier B.V. All rights reserved.