A recent report on the correlation between enhanced polymer mobility and ionic conductivity at room temperature in plasticized polyether-urethane solid polymer electrolytes (Forsyth ct al.[1]), has prompted the present investigation. Positron annihilation lifetime spectroscopy (PALS) has been used to study the effect of plasticizer addition on the room temperature free volume characteristics of the crosslinked polyether-urethane. The addition of low molecular weight plasticizers to the polyether-urethane results in a constant or decreasing mean free volume cavity radius, as measured by the ortho Positronium lifetime tau(3), and a decreasing relative concentration of free volume cavities as measured by the ortho-Positronium intensity, I-3. It is postulated that the plasticizers interrupt polymer-polymer interactions by occupying the inter- and intra-chain free volume. The plasticizer structure influences the polymer-plasticizer interactions which affect inter- and intra-chain separation and hence the free volume of the system. The decrease in polymer-polymer interaction and the increase in polymer-plasticizer interaction in turn influence the glass transition temperature behaviour.