A detailed study of the microstructure, the transport properties and the state of water for fluorine-free sulfonated poly(ether ether ketone) (s-PEEK) membranes with a degree of sulfonation of about 50% is reported as a function of the membrane water content. SAXS curves show how 50% s-PEEK membranes at low and intermediate water contents are characterized by a poor hydrophobic/hydrophilic phase-separation and narrow ionic domains (<2 nm). For membrane water contents of up to 40%, all the water molecules contained in the membranes appear to be strongly bounded to the sulfonic groups and water molecules are unable to freeze. When the membranes are treated in water at temperatures higher than 80 degrees C, membrane swelling starts and for water contents above 40-50% a better ionic microstructuring inside the membranes occurs. In highly swollen membranes part of the water molecules is in a less associated freezable bounded state. The presence of more mobile water molecules is responsible for the increase in the membrane proton conductivity and conductivity of the order of 0.1 S/cm has been recorded for s-PEEK with 69% of water content. (C) 2013 Elsevier B.V. All rights reserved.