Conducting properties of solid polymer electrolytes (SPEs) based on acrylonitrile copolymer (with 93 wt.% acrylonitrile units) and LiClO4 have been studied by a complex impedance and potentiostatic techniques. The results indicate existence of a highly ionically-conductive concentration region close to the limit of salt solubility. The formation of an ionic structure favourable for fast Li-ion transport is governed mainly by the following factors: absence of residual low molecular weight solvent (illustrated by conductivity and lithium transport numbers measurements) and temperature (IR spectroscopy investigations) influence.