Single-ion conducting polymer electrolytes have received great attention due to their potential applications in electrochemical energy sources. In this work we have in detail studied features of ion transport of alkali metal ions in the polymer electrolytes based on Nafion plasticized with DMSO. The samples have been characterized by DSC, FTIR, and impedance spectroscopy. Based on the obtained results the speculations about the reasons for the presence of extremes in dependencies of both ion conductivity and activation energy on cation radius have been made. It is also suggested that solvate Li+(DMSO)(4) is the minimum transport unit in Nafion-Li+ swollen in DMSO. (C) 2016 Elsevier B.V. All rights reserved.