Small-angle X-ray scattering and wide-angle X-ray scattering studies were carried out for perfluorinated sulfocationic membranes MF-4SK (membranes of a Nafion type) in the normal 'as-manufactured' state and soaked in ethylene glycol at 110degreesC and subsequently washed with water. It was shown that such a treatment of MF-4SK membranes resulted in alterations of their nanostructure and these alterations were stable for a long time. On the basis of the paracrystalline layered model of the perfluorinated membrane nanostructure these structure alterations were interpreted as an increase in the thickness of the liquid layers (ionic channels) separating clusters of ionomer groups in MF-4SK membranes. The importance of these structure alterations was confirmed by the twofold growth of the quantum yield of anthracene photo-oxidation catalyzed by tetraphenylporphyrin (TPP) immobilized on MF-4SK membranes treated with ethylene glycol. (C) 2003 Elsevier Ltd. All rights reserved.