The hydration in liquid water of various thermally treated SPOOK membranes was determined and confronted with data in saturated water vapor. Significant differences, which Correspond to the so-called Schroder paradox, are observed only in un-crosslinked ionomers with IEC > 2 eq/kg. After cation exchange in electrolyte solutions (2 and 0.1 mol/L), the hydration and density of the ionomers was determined and confronted with a physical model. There is 110 clear Correlation with cation properties, such as ionic radius or Robinson-Stokes hydration numbers. The ionic conductivity ranges from 3 mS/cm for K, NH4 and Mg-exchanged membranes to 0.6 mSicm for Ca and Zn. Although the latter might be related to ionic cross-linking by the divalent cations, the relatively high conductivity of Mg ions might be useful for electrochemical applications. (C) 2014 Elsevier By. All rights reserved.