The influence of surface-modified Al2O3 particles on the formation of different ionic aggregates in poly (ethylene glycol) methyl ether (PEGME)-LiClO4 and poly (ethylene oxide) dimethyl ether (PEODME)-LiClO4 electrolytes is discussed. Three independent methods have been used to estimate the fractions of free ions and ionic associates. The first two methods are based on the deconvolution of the FTIR 624 cm(-1) and Raman 930 cm(-1) perchlorate anion modes. The third method uses a Fuoss-Kraus semiempirical method involving the salt concentration dependence of ionic conductivity. Results are compared for two polyether systems to explain interactions in polymer electrolytes. based on low molecular weight polyglycols. The temperature dependence of the fractions of ionic species is also analysed. (C) 2004 Elsevier B.V. All rights reserved.