Highly concentrated polymer electrolytes based on poly(ethylene oxide) (PEG) and LiI, with EO/Li ratio less than or equal to 3, were investigated by differential scanning calorimetry (DSC), powder X-ray diffraction (XRD) and Li-7 solid state nuclear magnetic resonance (NMR) methods. The effect of 15-nm particle size Al2O3 additives and in several cases, other constituents ethylene carbonate and poly(methylmethacrylate) on structure and Li+ ion environment was explored. The addition of Al2O3 suppresses the formation of crystalline phases, including free LiI, which is present in EO/Li= 1.5 samples without Al2O3. The conductivity jump observed in these concentrated electrolytes at around 80 degrees C is correlated with an NMR-observed transition to a Li+ environment which is similar to that of free ions in a molten phase.