The interaction of the perfluorosulfonic membrane Nafion with H2O, CH3OH, (CH3)(2)O, and Py gives protonated monomeric and oligomeric species characterized by well-defined IR spectra. The proton-transfer process is also associated with a deep modification of the IR and Raman spectra of the polymer because of the disappearance of the vibrational modes of the undissociated -SO3H groups and of the appearance of the bands of the -SO3- species. The unique possibility afforded by the adopted in situ dosage technique of changing in a selected way the ratio between the monomeric and the polymeric species (during the adsorption and the desorption experiments) has allowed the assignment and the comparison of the IR spectra of the individual species to be made. These spectra compare very well with those of the same species in solution. The case of CH3CN is peculiar, as the proton transfer is clearly observed only at the highest dosages with formation of nitrilium species. These species do not find close and well-studied homogeneous analogues.