Perfluorosulfonate ion-exchanged membranes (PFSIEMs) have been prepared by the melt-extrusion method. Subsequently, the effect of annealing on the properties of PFSIEMs was studied. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) analysis suggested that -SO3 groups could be brought to the membrane surface, presumably by side chain movement during the course of thermal annealing. In the presence of coexisting ion clusters and crystallization, area resistance increased with the rise of the treatment temperature. X-ray diffraction (XRD) analysis further showed that the annealing treatment should increase the crystallinity of membranes, markedly. The equivalent weight (EW) and water uptake were also employed to evaluate the effect of annealing on membranes. For membranes prepared by melt extrusion, abnormal fluctuations are solely evident in the thermogravimetric (TG) traces of annealed specimens. In addition, two glass transition temperatures, ca. 120 and ca. 250 degrees C, have been observed in the differential scanning calorimetry (DSC) analysis. When the thermal annealing temperature was increased, however, the lower endothermic peak shifted to the high temperature region, mostly because water molecules can serve as the plasticizer to reduce T-g. In addition, we also found that the dynamic mechanical property of membranes can be substantially impacted by annealing. Changes in the temperature relaxation have been explored with dynamic mechanical analysis (DMA).