The microscopic and bulk swelling of "Nafion(R)" perfluorinated ionomer membranes in mixtures of ethanol and water was studied using small angle X-ray diffraction and optical microscopy. The microscopic swelling decreased with increasing ethanol content, in contrast to the bulk swelling which increased dramatically, reaching a maximum in a mixture containing 75% ethanol and 25% water. Uniaxially oriented membranes swollen with such an optimal mixture were found to relax back to an almost isotropic state, which could not be achieved by using water alone. The conclusion is that the ethanol plasticises the fluorocarbon matrix in Nafion, allowing the ionic material to be redistributed into smaller, more numerous clusters than in membranes swollen with water alone.