Positron annihilation lifetime spectroscopy (PALS) has been used to probe the effects on free volume of the addition of either propylene carbonate or tetraglyme to a polyether-based electrolyte. Despite their very different behaviour as observed by NMR and conductivity measurement, the PALS sensitive free volume of the samples changes in a similar way on addition of the two plasticizers. It is concluded that the effect of these plasticizers on conductivity is determined more by their effect on ion-polymer and ion-ion interactions than by their effect on the PALS free volume.