The supercritical CO2 (sc-CO2) provided a moderate condition to make the amorphous CO2/poly(ether ether ketone) (PEEK) mixtures at 30 MPa and 40 degrees C. The crystal is obtained directly after treating CO2/PEEK mixture from 70 to 240 degrees C. The crystallization behavior of CO2/PEEK mixtures before and after treatment is investigated in detail by using differential scanning calorimetry (DSC), dynamic mechanical analysis, and wide-angle X-ray diffraction. DSC curves of CO2/PEEK samples showed the double cold crystallization peaks. The lower cold crystallization peak moves to higher temperature with the content of CO2 decreasing, and the higher cold crystallization peak keeps their temperatures at about 172 degrees C without a remarkable change. The dynamic mechanical spectrometry was also introduced to explain the relaxation behavior of the glass transition and crystallization. (C) 2007 Wiley Periodicals, Inc.