Plasticized PEO based electrolyte membranes of various compositions, xPEO(1 - x)[LiCF3SO3/Dibutyl phthalate], where x varies from 0.17 to 0.84 were prepared by solution casting. The concentration of salt (LiCF3SO3) in plasticizer, dibutyl phthalate (DBP) was fixed at 1 mol/kg. The thermal properties, electrical conductivity and vibrational spectra were studied. DSC revealed the plasticizing effect of PEO for large amounts of plasticizer added DBP (50 wt%). Thermal events such as melting and glass transition were affected if the dissolution of components in solvent and drying of membrane was altered in terms of duration. Complexation of salt to polymer was substantiated by the appearance of new bands not present in PEO/plasticized PEO and also broadening of the C-O-C vibrations as the salt content increased. Room temperature conductivity changed over 3 orders of magnitude, increasing from 7.1 X 10(-7) S cm(-1) for x = 0.84 to 6.0 x 10(-4) S cm(-1) for x = 0.33. This conductivity is much higher than in similar PEO-Lithium salt systems that are not plasticized. By correlating the trends in T-g and conductivity with composition, it appears that the enhancement can be attributed to, primarily, an increase in carrier concentration. The plasticizer seems to play a catalytic role in dissociating the salt and increasing the carrier concentration. It is speculated from the present study as well as observations of a similarly plasticised system, xPVC(1 - x)LiN(CF3SO2)(2)/DBP which was investigated in detail, earlier, that the inclusion of the plasticizer, DBP in conventional polymer-Li salt changes the solvation mechanism.