The products of solvation of lithiated Nafion with dimethylsulfoxide (DMSO) have been studied by ATR IR spectroscopy in the frequency range of 50-4000 cm(-1) in a vacuum at room temperature. Degree of solvation n = DMSO/Li has been varied in a range of 1.5-18.4. Based on analysis of the dependence of the spectral parameters on the n value, it has been concluded that the test samples contain DMSO molecules of two types. The first type includes molecules coordinated to the lithium ion through the formation of the metal-oxygen bond, and the second is DMSO molecules associated by intermolecular bonds similar to those in the liquid DMSO phase. The structure of the salt depends on n: the changes are attributed to the reorientation of polymer chain units. The coordination number of lithium has been estimated at four. It has been shown that the IR data are consistent with the data on the conductivity of lithiated Nafion membranes as a function of DMSO content, according to which the simplest transport unit is the tetrasolvate [Li(DMSO)(4)](+). At temperatures below 0A degrees C, all the samples exhibit an abrupt change in conductivity, which is attributed to the freezing of DMSO in the membrane matrix.