Fourier transform infra-red (FTIR) spectroscopic studies have been carried out to probe, at the microscopic level, the effect of filler concentration on poly ethylene oxide (PEO) based composite polymer electrolytes complexed with sodium salts (NaClO4 and Nal) dispersed with inorganic fillers, Na2SiO3 and SnO2. The FTIR results correlate very well with macroscopic changes such as glass transition temperature (T-g), electrical conductivity and mechanical properties in the composite polymeric systems due to the dispersion of filler particles. The striking observation of the two maxima feature in electrical conductivity (sigma) variation versus filler concentration is satisfactorily explained in terms of variation of T-g and degree of crystallinity of the polymer composites. Although, few such observations have been reported by other earlier workers in some composite systems no explanation is available at the microscopic level. A mechanism at microscopic level from FTIR results involving ion-polymer, ion-ion, ion-filler and polymer-filler interactions has been proposed by us to explain the variations of sigma and T-g as a function of filler concentration. The initial increase in free anions due to the interaction of fillers with ether oxygen of PEO and/or ClO4-anions followed by ion-pair formation results in the first maxima in the variations of sigma vs. filler concentration. The second maxima results from the additional generation of free anions due to weakening of filler-anion interactions followed by the cluster formation on addition of fillers. The improvements in mechanical properties of the composite polymer electrolytes have also been explained on the basis of the proposed mechanism. (C) 2010 Elsevier B.V. All rights reserved.