The effects of salt (LiClO4) concentration and plasticizer (tetraglyme) concentration on the room-temperature conductivity and free volume of a polyether-urethane solid polymer electrolyte are studied. The free volume is probed by the positron annihilation lifetime spectroscopy (PALS) technique which uses the oPs pick-off lifetime, tau 3, as an indication of the local electron density and the mean free volume cavity radius. The oPs pick-off intensity, I3, reflects the probability of oPs formation and the relative concentration of free volume cavities in the material. The mean size ( tau 3) and relative number (l3) of free volume cavities decrease with an increase in salt concentration in the host polymer due to the Li+ coordination (effective cross-linking) with the oxygens of the host polymer. The addition of 25 wt% tetraglyme plasticizer to the 1 molal LiClO4/host polymer complex is shown to counter the effective cross-linking resulting in a Tg decreased to a value equal to that of the pure host polymer, increased conductivity, and average free volume cavity size ( tau 3) increased to a value equal to that of the pure host polymer. However, the relative number of free volume cavities (l3) in the plasticized host polymer/salt complex remains fewer than that of the pure host polymer over the concentration range of plasticizer studied, and in a similar manner the density remains greater than that of the pure host polymer. The room-temperature conductivity, free volume, and density behaviour in conjunction with the Tg results suggest that the plasticizer addition leads to Li+ coordination with the oxygens of the plasticizer chains as well as increased mobility of the host polymer chains. The increase in free volume of the host polymer/salt complex caused by the addition of plasticizer is shown to dominate the conductivity behaviour in this system.