A perfluorosulfonic acid (PFSA) polymer with pendant side chain -O(CF2)(4)SO3H was doped with the heteropoly acids (HPAs), H(3)PW(1)2O(40) and H4SiW12O40. Infrared spectroscopy revealed a strong interaction between the HPA and the PFSA ionomer. Modes associated with the peripheral bonds of the HPA were shifted to lower wave numbers when doped into PFSA membranes. Small-angle X-ray scattering (SAXS) measurements showed the presence of large crystallites of HPA in the membrane with d spacings of ca. 10 angstrom, close to the lattice spacing observed in bulk HPA crystals. Under wet conditions the HPA was more dispersed and constrained the size of the sulfonic acid clusters to 20 angstrom at a 5 wt% HPA doping level, the same as in the vacuum treated ionomer samples. Under conditions of minimum hydration the HPA decreased the E. for the self-diffusion of water from 27 to 15 kJ mol(-1) (.)The reverse trend was seen under 100% RH conditions. Proton conductivity measurements showed improved proton conductivity of the HPA doped PFSAs at a constant dew point of 80 degrees C for all temperatures up to 120 degrees C and at all relative hummidities up to 80%. The activation energy for proton conduction generally was lower than for the Undoped materials at RH < 80%. Significantly the E. was 1/2 that of the undoped material at RHs of 40 and 60%. A practical proton conductivity of 113 mS cm(-1) was observed at 100 degrees C and 80% RH. (c) 2007 Elsevier Ltd. All rights reserved.