Solid polymer electrolytes based on poly(ethylene glycol)-polyurethane (PEG-PU) complexed with LiClO 4 salt have been prepared by the solvent casting method. A PEG-PU material (PEG:4,4'-diphenylmethane diisocyanate:1,4-butanediol = 1:2:1) was synthesized through a typical two-step condensation reaction. We investigated the effects of the salt concentration on the ionic conductivity (σ) and the glass transition temperature (T g ) of the complex electrolytes by using alternating current impedance spectroscopy, differential scanning calorimetry, and dynamic mechanical thermal analysis. The measured values of both σ and T g exhibited similar tendencies in that they had maxima within the range studied, probably because of two opposite effects, i.e., the increased number of carrier ions and the decreased chain mobility (or increased T g ) caused by the increase in the salt concentration. The highest conductivity, on the order of 2.43 ×10 6 Scm -1 , was obtained at an [O]/[Li + ] ratio of ca. 16 (0.92 mol salt per kg of matrix polymer).