Polymer electrolytes are attracting interest because of potential use in ‘solid state’ batteries and electrochromic windows. In this work the nature of the chemical bonds and ion association in polymer-salt complexes were studied. The fundamental vibrational frequencies of LiClO4− were calculated using valence force constants which are obtained from the ClO4− anion. The calculated frequencies and the band assignments agree quite well with the observed frequencies. Raman and IR spectroscopy were used to investigate the interaction of Li+ and ClO4− with the PEO chains. Lithium ion interacted with the negatively charged oxygen of PEO and C104 interacted electro- statically with the positively charged hydrogens. The strong electrostatic interactions induced changes in the intensity of the Raman bands and IR absorption bands of the polymer. The Raman and IR spectra of the polymer-salt complexes clearly show the existence of ‘free’ ions, ionpairs and salt clusters at various concentrations of the lithium salt. The maximum concentration of “free” ions deduced from the spectroscopy is in excellent agreement with the salt concentration required to obtain maximum ionic conductivity.