The salt diffusion coefficient in a series of nanostructured block copolymer electrolytes was measured in a symmetric lithium/polymer electrolyte/lithium cell using the restricted diffusion technique. The decay of the open-circuit potential as measured by this technique was analyzed by a Laplace inversion algorithm to give the distribution of relaxation processes characteristic of the electrolytes. The distribution function was characterized by two parameters, an average diffusion coefficient, D-avg, and a polydispersity index, PDIdiffusion, which is a measure of the width of the distribution. We compare these parameters obtained from a series of nearly symmetric poly(styrene)-block-poly(ethylene oxide) (SEO) block copolymer electrolytes containing lithium bis(trifluoromethanesulfone) imide salt (LiTFSI) with those obtained from a homogeneous poly(ethylene oxide) (PEO)/LiTFSI mixture. D-avg of the SEO/LiTFSI mixtures increases with increasing molecular weight of the PEO block, M-PEO, and reaches a plateau of 2/3D(PEO) when M-PEO exceeds 50 kg/mol (D-PEO is the average salt diffusion coefficient in PEO homopolymer). The PDIdiffusion values obtained for SEO copolymers are significantly higher than those obtained in PEO homopolymer. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3563802]