The water sorption into a polysulfone membrane aminated with trimethylamine has been studied by means of attenuated total reflection Fourier transform infrared and Raman spectra. Spectra of model compounds diphenyl sulfone, diphenyl ether, diphenylisopropane, p-toluenesulfonyl chloride, and tetramethylammonium chloride are used to assign the sulfo, ether, and quaternary amine site contributions to infrared and Raman spectra. The polymer-water interaction causes systematic decrease of CH stretching intensities in both IR and Raman spectra. Intensity changes are explained by the ordering of water around quaternary amine sites and by the weakening of the interaction between the positively charged sites and the counterions. Sorption of water into the membrane also causes systematic changes of modes related to sulfo and ether groups. Slight shifts toward longer wavelengths and changes of the band shape are observed, indicating decrease of interchain interactions in the polysulfone membrane.