The work addresses the properties of electro-transport in highly ion-conductive polymer electrolytes based on Nafion-115 membranes in the NH4+ ionic form plasticized with dimethyl sulfoxide (DMSO). The ionic conductivity and the activation energy of the conductivity were found to exhibit three types of behavior as a function of the plasticizer. The first region (when the ratio of moles of DMSO molecules per mole of NH4+ (n) < 6) is characterized by low conductivity values (10(-7)-10(-4) S cm(-1)) and high energy barriers (0.76 and 0.43 eV for n = 0 and 2.6, respectively). In the second region (n = 6-12), the electrolyte has a high conductivity and a low activation energy of conductivity (similar to 0.1 eV). With an increase in the DMSO content, the conductivity appreciably increases and reaches similar to 0.4 mScm(-1). In the third region (n = 12) the transport parameters are comparable to those of the second region, but the conductivity is practically independent of the DMSO content. The observed behavior was explained on the basis of the results obtained with the help of IR spectroscopy, differential scanning calorimetry, small-angle X-ray scattering and quantum-chemical modeling. (C) The Author(s) 2019. Published by ECS.