The usefulness of x-ray photoelectron spectroscopy to study surface structural aspects of polymer electrolytes has been demonstrated by its application to complexes of poly(ethylene oxide) with lithium perchlorate or copper perchlorate in various molar ratios. The binding energies for various core level electrons of the different elements present in these electrolytes have been determined. The shifts in observed binding energies with respect to the elemental values are interpreted by considering the effect of the surrounding ions’ electronegativity on the core electrons. These results present direct evidence for the interaction between PEO and the cation/anion of the salt, caused by the complexation between the oxygen of the -CH2-CH2-O- repeating units and the cations of the salt as well as the effective cross-linking between the polymer chains of PEO through the anions of the salt. While in neat PEO there is only one type of carbon atom, the above interactions with the salt lead to the formation of nonequivalent carbons. These interactions are described by the varying cation environment and the unchanged anion environment for the different PEO-salt complexes.