The ionic structure and transport properties intrinsic to solid polymer electrolytes based on an acrylonitrile–butadiene (40 : 60) copolymer and lithium hexafluoroarsenate are studied at the salt concentrations close to the salt solubility limit in the polymer matrix. The ionic structure of a macromolecular ionic solution alters with increasing salt concentration and temperature, as does the ionic-transport mechanism. Possible models for the ionic structure of concentrated solid polymer electrolytes and ionic-transport mechanisms corresponding to them are considered.