A blend of poly(methyl methacrylate) (PMMA) and poly(styrene-co-acrylonitrile) (PSAN) has been evaluated as a composite polymer electrolyte by means of differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, ac impedance measurements, and linear sweep voltammetry (LSV). The blends show an interaction with the Li+ ions when complexed with lithium perchlorate (LiClO4), which results in an increase in the glass-transition temperature (T-g) of the blends. The purpose of using PSAN as another component of the blend is to improve the poor mechanical properties of PMMA-based plasticized electrolytes. The mechanical property is further improved by introducing fumed silica as inert filler, and hence the liquid electrolyte uptake and ionic conductivity of the composite systems are increased. Room-temperature conductivity of the order of 10(-4) S/cm has been achieved for one of the composite electrolytes made from a 1/1 blend of PSAN and PMMA containing 120% liquid electrolyte [1M LiClO4/propylene carbonate (PC)] and 10% fumed silica. These systems also showed good compatibility with Li electrodes and sufficient electrochemical stability for safe operation in Li batteries.