A difference-spectrum method of FTIR was used to study the variation of characteristic infrared spectral absorption band in carboxylated PPO and polystyrene blends. It was found that vibration frequency shifts of the characteristic infrared absorption bands in CPPO /PS blends

with 8.0% carboxylated contents in CPPO and 50 / 50 ratio of CPPO /PS

varied regularly with raising temperature. It was found that three slopes corresponding to the three temperature ranges: below 190℃

190～220℃ and above 220℃

appeared in frequency-temperature curves for the CPPO/PS blend. These temperature ranges are consistent with the UCST area

the miscible area and the LCST area abtained by our previous works.The variation of interaction between CPPO and PS molecules with raising temperature was studied.The mechanism of phase transformation was discussed.