The behavior of positrons that do not form positronium in dry fluorinated polymer electrolyte membranes (Nafion (R), Fumapem (R) and Aquivion (R)) with various ion exchange capacities (IECs) was studied by the combined use of Doppler broadening of annihilation radiation (DBAR) and the positron lifetime technique. The drastic increase of the S parameter, measured by DBAR, with increasing IEC above 0.91 meq/g indicates that increasing numbers of positrons are trapped by oxygen atoms and annihilate with the electrons bound in them. Reversed micelle like SO3H nanoclusters to trap positrons possibly appear at IEC = 0.91 meq/g and their concentration increases with increasing IEC. (C) 2012 Elsevier B.V. All rights reserved.