The addition of low molecular weight solvents such as dimethyl formamide (DMF) and propylene carbonate (PC) to urethane crosslinked polyethers results in enhancement of polymer segmental motion, as determined in this work from polymer C-13 spin lattice relaxation measurements (T-1) and glass transition temperatures. The formation of salt-polyether complexes results in a decrease in T-1, even in the presence of the plasticizer, indicating that the polymer ether molecules are still involved in the alkali metal coordination. In a polymer electrolyte containing 1 molkg(-1) LiClO4 the addition of DMF and PC have significantly different affects on the polymer mobility, although they both enhance the conductivity. The conductivity enhancement therefore is not solely the result of an increased solvent mobility.