Water sorption, volumetric expansion, and proton conductivity of 1100 EW Nafion and 555 EW sulfonated polyetheretherketone (SPEEK) were compared as functions of water activity at 60 and 80 degrees C. Water sorption in Nafion occurs with a small positive volume of mixing, similar to 0.005 cm(3)/cm(3). In contrast, water sorption in SPEEK has a large negative volume of mixing similar to-0.05 cm(3)/cm(3). The percolation thresholds for proton conduction occur at hydrophilic volume fractions of 0.10 in Nafion and 0.30 in SPEEK. Proton conductivity increases quadratically with hydrophilic volume fraction above the percolation threshold. The different percolation thresholds suggest the hydrophilic domains in Nafion grow from lamella, whereas the hydrophilic domains in SPEEK grow from spheres. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1437-1445, 2011