We have studied water contained in the perfluorinated sulfonic acid (denoted as RSO3H) membrane by FTIR spectroscopy. The IR spectra:of the membrane, which absorbed water quickly even from the air,, changed so, diastically during dehydration that they showed clearly two isosbestic points in the 4000-1500 cm(-1) region. Analysis of the IR spectral changes made it clear. that water molecules exist mostly as H3O+ in equilibrium with RSO3-and RSO3H in,the membrane. The IR change accompanying dehydration was found to. result from the reaction as RSO3- + H3O+ --> RSO3H + H2O(evaporating out). The IR spectrum of H3O+ was clearly separated, which shows a broad OH stretching absorption. with two unseparated peaks around 3370 and 3120 cm(-1) and an OH deformation absorption at about 1720 cm(-1). The spectrum is distinctively different from that of water, separated, coexisting with H3O+, RSO3-, and RSO3H in the membrane, which shows OH,stretching and deformation absorptions at about 3480 and 1628 cm(-1), respectively.