Abstract: The effect of lithium salt concentration on ion-molecular and ion-ion interactions in amorphous solvent-free polymer electrolytes was studied by FTIR spectroscopy and quantum chemical calculations. The study was carried out on the elastomeric random copolymer of butadiene and acrylonitrile (PBAN) containing LiAsF6 over the range 0.05-1.89 mol kg- 1 (the limit of the homogeneity region of the system). Calculated IR spectra for stable clusters Li+m[AsF6]-n with predicted geometry were used as patterns to establish predominant ion species in PBAN-LiAsF6. It was found that a number of concentration regions corresponding to distinct predominant species existed within the homogeneity region. At low salt content, a series of equilibria between contact ion pairs and free ions was established. Li+ coordination with respect to the octahedral [AsF6]- in ion pairs changes from bidentate to tridentate, and equilibrium shifts to the ion pairing as concentration increases. At moderate concentrations, the tridentate ion pairs are the only predominant species. At high concentrations, neutral aggregates {Li+[AsF6]-}n (n = 2-4) dominate. Highly concentrated electrolytes were found to be metastable, and predominant species were determined by prehistory of a film rather than by concentration. Any ion species (with the exception of free [AsF6]-) are solvated by nitrile groups of macromolecules. The apparent solvation number of Li+ calculated from IR results varies with concentration, in accordance with changes in ion association. Long-term storage is needed to reach equilibrium or quasi-equilibrium state of PBAN-LiAsF6. © 2007 Elsevier B.V. All rights reserved.