Characterization and interaction behavior between Li+ ion and C=O groups of a series polyester electrolyte have been thoroughly examined using Fourier transform infrared (FTIR). The "free/Li+ bonded" C=O absorptivity coefficient of the LiClO4/polyester can be determined quantitatively using FTIR spectrum ranging from 1800 to 1650 cm(-1) at 80 degrees C. Results from curve fitting show that the "free/Li+ bonded" C=O absorptivity coefficient is 0.144 +/- 0.005. The C=O group of polymer electrolyte shows strong interaction with Li+ ion and a limit value of 95% "Li-divided by bonded" C = O is approached in the polymer electrolyte system when the Li+ ion equivalent fraction is about 0.28. The molecular structure of polyester electrolyte does not affect significantly the efficiency of interaction between Li+ ion and C=O. (c) 2007 Published by Elsevier Ltd.