The thermally-induced phase transformation behaviors of carboxylated poly(phenylene : oxide)(CPPO)/polystyrene(PS), and PPO/PS blends were studied by using a difference-spectrum method of Fourier Transform Infrared Spectrometry (FTIR). The difference intensities(mu) and vibration frequency shifts(Delta upsilon) of the characteristic infrared absorption bands in both CPPO/PS and PPO/PS increased with raising temperature. A linear temperature dependence in the band frequency vs. temperature plots was observed for PPO/PS system, while three slopes corresponding to the three temperature ranges : below 190 degrees C, 190 degrees-215 degrees C, and above 220 degrees C, 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 obtained by our previous works. The mechanism of phase transformation of CPPO/PS was discussed.