The plasticized polymer electrolytes composed of acrylonitrile (AN)-methyl methacrylate (MMA) copolymer and liquid electrolyte solution have been prepared. The electrochemical characteristics of these polymer electrolytes were investigated in terms of copolymer composition and content of liquid electrolyte. The relative molar ratio of AN-NMA in the copolymer proved to play a critical role in determining the ionic conductivity and capacity to retain electrolyte solution. The ionic conductivities of these systems reached an order of 10(-3) S/cm in the presence of 80 wt.% of liquid electrolyte solution at room temperature, giving homogeneous films which exhibited good mechanical properties. The encapsulation of liquid electrolyte in the matrix polymer was found to diminish the passivation of lithium electrodes. The polymer electrolytes studied in this work displayed a sufficient electrochemical stability to allow safe operation in rechargeable lithium polymer batteries.