This work aimed at studying the efficiency of the use of supercritical carbon dioxide (scCO(2)) as a swelling agent to disperse sodium benzoate (NaBz), a nucleating agent, in polypropylene (PP), on the one hand; and the crystallization behavior of the resulting PP, on the other hand. Under scCO(2), the NaBz was uniformly dispersed in the PP at a nanometer scale. The use of ethanol or acetone as a cosolvent further increased its state of dispersion and its mass uptake in the PP. Isothermal and nonisothermal crystallization kinetics indicated that the PP with the NaBz being dispersed in it under scCO(2) had a much higher crystallization rate than that of the pure PP or the PP with the NaBz being dispersed in it by a conventional melt compounding process. The size of the PP crystallites was also much smaller when the NaBz was dispersed at a nanometer scale. (c) 2006 Wiley Periodicals, Inc.