Complex impedance studies have been carried out on Dow 800, Dow 1000 and Nafron 117 membranes at various water contents and a variety of temperatures and hydrostatic pressures. At room temperature and pressure the usual gradual decrease in electrical conductivity with decreasing water content is observed. For very low water content materials the variation of the conductivity with pressure from 0 to 0.2 GPa (2 kbar) is large and gives rise to apparent activation volumes, DeltaV as large as 54 cm(3)/mol. in addition, for low water content materials, there is a tendency for smaller equivalent weights (same side chains) or larger side chains to have larger activation volumes. At high water content, DeltaV is relatively independent of the host polymer and negative values are observed at the highest water contents. These results provide support for the model where proton transport in high water content sulfonated fluorocarbons is similar to that for liquid water. All results are explained qualitatively via free volume. Ambient-pressure, variable-temperature H-2 T-1 and linewidth measurements imply a heterogeneous environment of the water molecules. Proton pulsed field gradient NMR studies in saturated Dow membranes verify the expectation that ionic conductivity is determined primarily by diffusion of water molecules. (C) 2000 Elsevier Science B.V. All rights reserved.