The hydration thermodynamics and transport properties of proton conducting ionomers and polyelectrolytes are commonly described using relative humidity RH and temperature T as the only variables. From the hydration behavior and the viscoelastic properties of Nafion (R) as a function of relative humidity RH and temperature T, this work provides evidence for the importance of internal pressure p as an additional degree of freedom. This is implemented into a phenomenological description of membrane swelling containing exothermal ion hydration, the entropy increase associated with the dilution of protonic defects within the aqueous domain and the effect of internal swelling pressure on the chemical potential of water as main ingredients. Since the internal pressure may be as high as 10 MPa (100 bar) it does not only have a significant effect on the chemical potential of water, but internal pressure gradients must also be implemented as a separate driving force for hydrodynamic water transport. The internal pressure responds to changes of the membrane's viscoelastic properties which are suggested to be different for bulk and an "extended layered surface skin". The latter is put forward to be very tough parallel and little permeable for water normal to the membrane surface. Changes of the anisotropic "skin" structure in contact with liquid water are suggested to release most of the internal pressure which may provide an explanation for the improved performance of certain composites and "Schroeder's paradox". The effect of internal pressure on water transport through and within Nafione is discussed as well. (C) 2013 Elsevier B.V. All rights reserved.