Two frequently neglected topics in the field of polymer electrolytes are discussed, namely the structure of crystalline polymer:salt complexes and the redox electrochemistry at the metal electrode/polymer electrolyte interface. The crystal structure of the PEO(3):LiCF3SO3 complex has been obtained from powder X-ray data. The PEO chains adopt a helical conformation with Li+ ions located in each turn of the helix and coordinated by three ether oxygens plus one oxygen from each of two CF3SO3- anions completing a trigonal bipyramidal coordination around the Lif ion. Each CF3SO3- anion bridges two adjacent Li+ ions along the chain, the -CF3 groups project into the interchain space. The polymer electrolyte may be regarded as an infinitely extended columnar coordination complex with no ionic crosslinking between the helical columns. Cyclic voltammetry and ac impedance measurements have been carried out on the Eu-3+/2+ couple in PEO at 80 degrees C. The voltammetric results are consistent with an EC mechanism, involving reduction of Eu3+ to Eu2+ followed by a chemical step in which Eu2+ is consumed. Both electron transfer and Eu3+ diffusion appear to be slow. Ac impedance suggests that D-Eu3+ is approximately 4 X 10(-16) cm(2) s(-1).