To investigate the solution-cast process from concentrated DMF-based perfluorosulfonic ionomer (PFSI) solution to membranes and morphology of the corresponding solution-cast membranes, XRD measurements were conducted on concentrated solutions, gels of various ionomer concentrations and membranes prepared at different temperatures, and small-angle X-ray scattering from the membranes was studied in order to investigate their internal supermolecular structure. It is with interaction of backbones of the ionomer molecules that the PFSI aggregates in the solutions fuse into each other to form gels and membranes, with capillary liquid bridge force and thermal movement of the polymer chains as the driving force on drying. Fusion degree of the ionomer aggregates increases with increasing casting temperature. Accordingly, proton conductivity, water uptake, solubility in ethanol/water mixture and crystallinity of the membranes decrease with increasing casting temperature, whereas dry density and tensile stress at break of the solution-cast membranes increase. SEM and TEM measurements were also conducted to give direct observation of the structure of the membranes. It was also found that morphology of DMF-based solution-cast PFSI membranes is greatly different from that of Nafion (R) thermo-extruded perfluorosulfonic membranes. (C) 2008 Elsevier B.V. All rights reserved.