Values for the lithium ion transference number ( ) are reported for the solid polymer electrolyte system poly(ethylene oxide) (PEO) complexed with Li(CF3SO2)2N (LiTFSI). ranges from 0.17 ± 0.17 to 0.60 ± 0.03 in the salt concentration (c) region of 742 to 2982 mol/m3 at 85 °C. The concentration dependence of and the molar ionic conductivity (Λ) are shown to be in good agreement with a free volume approach over the salt-rich composition range investigated. The present results were obtained using an electrochemical technique based on concentrated solution theory. This experimentally straightforward method is herein demonstrated to give accurate results for a highly concentrated SPE system, without relying on any dubious simplifications regarding the state of the electrolyte.