Solid polymer electrolytes in the system PEOnLiNTf2 (NTf2=N(CF3SO2)2; 3 <= n <= 20) were synthesised and characterised employing DSC, Xray diffraction and a range of solid state NMR methodologies including Li-7- and F-19 (MAS) NMR allowing to distinguish and quantify mobile and immobile cationic and anionic species, temperature dependent C-13 CPMAS NMR to identify amorphous and crystalline regions and C-13_ {Li-7} REDOR NMR to study the interaction between the Li cations and the polyether chains in the various identified phases. Significant ion mobility was found for compositions with EO:Li >= 10 only, with NTf2 anions being more mobile than lithium cations. While PEO3LiNTf2 and PEO6LiNTf2 mainly consist of the stoichiometric crystalline complexes, the dilute compositions show a complex phase behaviour. The results suggest that in this regime, pristine PEO, crystalline PEO6LiNTf2 and amorphous PEO6LiNTf2 coexist. The combined results from temperature dependent C-13-CPMAS and REDOR NMR experiments performed at temperatures below the glass transition temperature indicate that PEO10LiNTf2 reversibly separates into crystalline PEO and PEO6LiNTf2 upon cooling. (c) 2008 Elsevier B.V. All rights reserved.