Quantum-chemical modeling of structure and cation migration barriers in Nafion-like ammonium substituted ionomers plasticized with dimethyl sulfoxide (DMSO) was investigated by ab initio calculations. We use B3LYP/6-31G* hybrid density functional methods and the PBE/PAW method taking into account the gradient corrections and periodic boundary conditions. It is shown that at a low content of DMSO (n <= 4), NH4+ cation removal from the So(3)(-) -group occurs with a significant energy cost (> 0.4 eV). As the amount of DMSO increases, both the separation energy and the barriers to ammonium ion migration decrease to 0.1-0.2 eV. Ab initio molecular dynamics modeling demonstrated that at a moderate temperature (similar to 350 K), there is a rapid (similar to 15 ps) redistribution of the DMSO molecules between the Nafion chains located at distances <= 2 nm.