The performance of a polymer electrolyte fuel cell is critically dependent on the water uptake in the polymer electrolyte, usually Nafion. Nafion in solution is often painted onto the electrodes of the fuel cell. It is important that this cast Nafion film stay amorphous and not crystallize. Cast Nafion films, cn. 1 mu m thick, crystallized on silicon plates when kept in air at room temperature for a long time. The films contain large crystalline regions ranging from 0.5 mm to several millimeters in size. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) microspectroscopy have been used to investigate the crystallins and amorphous regions. The XRD shows two sharp peaks. One of the peaks is developed before the second one appears in the diffractogram, indicating that there might be two types of crystallizing processes. FTIR spectra of the amorphous and crystalline regions in the films show important rant differences. In the crystalline regions, the Film contains the sulfonic acid at the end of the side chains; hence, the crystalline regions contain no water molecules. In the amorphous regions there is a complete proton transfer from the acid to the water mol molecules, and sulfonate groups are obtained.