Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy was used to study in detail water vapor sorption in a short-side-chain perfluorosulfonic acid ionomer membrane suitable for use as electrolyte in proton exchange membranes fuel cells. The analysis of the membrane IR spectra, at different values of relative humidity (0.00-0.50) and at 35 degrees C, allows to identify four types of water molecules, characterized by decreasing strength of interaction with the polymer sulfonate groups. The actual concentration of the different water species inside the membrane was determined by calibrating the IR absorbance data with independent measurements of total water vapor uptake. The sorption of the different populations of water can be represented by Langmuir isotherms: the first population is directly attached to sulfonate sites, while the others form subsequent layers, adsorbed one onto the other in a shell-like structure. To describe the overall sorption behavior of the different populations, four adjustable parameters are required, which are consistent with literature data, thus supporting the validity of the physical model considered.