A detailed study of the dehydration process of hydrated Nafion-Na membranes has been performed using thermogravimetric analysis (TGA) and time-dependent (TD) FTIR, with the aim of investigating the water loss process during dehydration. The TGA data provide the absolute water weight loss, and TD FTIR spectra yield the relative IR intensity loss. Both TGA profiles and TD FTIR traces can be well fitted by second-order exponential decay, which implies two different dehydration processes, depending on water content. The fast process is attributable to high water level Nafion-Na (HWLN), with percolation structure and thus higher water diffusivity. The slow process is characteristic of low water level Nafion-Na (LWLN), with specific isolated cluster structure and lower diffusivity. For LWLN, the relative IR intensity loss of water bands is significantly faster and more extensive than the corresponding relative weight loss obtained by TGA. This deviation seriously calls into question the use of IR spectra as a quantitative measurement of water content in Nafion. The apparent dependence of the IR spectrum on the size of the nanoscale water clusters is in agreement with Gebel's mechanism of water swelling in Nafion.