In this study, dilute Nafion solutions consisting of solvents with various dielectric constants epsilon and solubility parameters delta, i.e. N,N'-dimethyl acetamide, N,N'-dimethyl formamide, N-methyl formamide, methanol-water mixture (4/1 g/g), ethanol-water mixture (4/1 g/g), and isopropanol-water mixture (4/1 g/g), were freeze dried and the conformations of Nafion molecules in dilute solutions were observed using transmission electron microscope. The membranes were prepared by solution casting from these solutions and evaporating the solvents at temperatures below T-G of Nafion, then annealing the membranes at 150 degrees C which was similar to 50 degrees C above T-G of Nafion. We show Nafion molecular conformations in dilute solutions are strongly influenced by delta and epsilon. of solvents. And, thus the morphology, water uptake, proton conductivity, and methanol permeability of membranes prepared by solution casting are also influenced by delta and epsilon. of solvents. (C) 2009 Elsevier Ltd. All rights reserved.