A novel class of esters of benzene 1,2 dicarboxylic acids such as dioctyl phthalate (DOP), dibutyl phthalate (DBP) and dimethyl phthalate (DMP) have been used as plasticizers in high molecular weight PEO-LiClO4 matrix (PEO= poly(ethylene oxide)) to improve the room temperature ionic conductivity of polymer-salt complex. Structural characterization was done by X-ray diffraction (XRD), thermal stability and glass transition properties were studied by DTA, and electrical (ionic) conductivities by ac-impedance spectroscopy. The XRD measurement ensures relatively low crystallinity when DOP is used as the plasticizer in the (PEO)(8)-LIClO4 matrix. It is found that the conductivity of 9.76 X 10(-5) S cm(-1) can be achieved at room temperature for the composition (PEO)(8)LiClO4:DOP (99.9:0.1 w/o) whereas it improves an order of magnitude (sigma = 4.29 X 10(-4) S cm(-1)) upon dispersing fine particles of gamma-Al2O3 (0.01 mu m) as inert fillers into the above plasticized polymer electrolyte matrix. Addition of DBP and DMP into PEO-LiClO4 electrolyte blend provides an ionic conductivity of similar to 10(-5) S cm(-1) at room temperature.