The ionic conductivity, σ, of mixtures of nearly symmetric polystyrene-block-poly(ethylene oxide) copolymers and Li[N(SO 2CF 3) 2] (LiTFSI) salt was measured as a function of molecular weight, salt concentration, and temperature. The molecular weight of the polyethylene oxide) block, M PEO, was varied from 7 to 98 kg/mol. The molar ratio of lithium to ethylene oxide, r, was varied from 0.02 to 0.10. In general, σ increases with increasing M PEO for all values of r. The data can be summarized by plots of normalized conductivity, σ n, versus M PEO, where σ n = σ/fφ PEOσ PEO), φ PEO is the PEO volume fraction in the copolymer, σ PEO is the conductivity of PEO homopolymer, and f is a morphology-dependent factor set equal to 2/3 for our lamellar samples. The temperature-dependent conductivity data at a given salt concentration collapse onto a single curve when plotted in this format. At r = 0.085, σn values reach a plateau in the vicinity of unity in the high M PEO limit. At other values of r, σn continues to increase with M PEO within the experimental range and reaches a value of around 0.5 in the high M PEO limit. © 2009 American Chemical Society.