Solid polymer electrolytes have been prepared by encapsulating plasticized polymer electrolytes based on poly(tetraethylene glycol diacrylate) into the pores of Celgard(8) microporous membranes. Electrolyte membranes with thicknesses of similar to 40 mu m have been prepared. The conductivity of the electrolyte is determined by the porosity of the membrane. and a conductivity of similar to 2 x 10(-4) Ohm(-1) . cm(-1) at room temperature has been demonstrated. These electrolytes showed good compatibility with Li and an electrochemical stability window spanning the range of 0.0 to similar to 4.5 V vs. Li+/Li. The excellent chemical and electrochemical stabilities enabling the use of the membranes in rechargeable Li batteries have been confirmed by the discharge/charge cycling of Li/solid polymer electrolyte/LiMn2O4 cells at room temperature.