The effect of CO2 on the nonisothermal crystallization of isotactic polypropylene (iPP) was studied with high-pressure differential Scanning calorimetry at cooling rates of 0.2-5degreesC/min. CO2 significantly delayed the melt crystallization of iPP, and both the crystallization temperature and the heat of crystallization decreased with increasing CO2 pressure. The crystallization rate of iPP, as characterized by the half-time, was also prolonged by the presence of CO2. With a modified Ozawa model developed by Seo, the Avrami crystallization exponent n of iPP was calculated. This value was depressed by the addition of CO2 and was strongly dependent on the CO2 pressure at low cooling rates. (C) 2003 Wiley Periodicals, Inc.