The isothermal crystallization kinetics of isotactic polypropylene (iPP) and iPP nucleated with the sorbitol derivatives 1,3:2,4-bis(4-methyldibenzylidene)sorbitol and 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol was studied, along with the subsequent melting behavior, as a function of the nucleating agent concentration. The influence of the agents on the crystallization rate, crystallization temperature, and crystallization range was examined. The isothermal crystallization temperature increased, along with the crystallization rate, with increasing nucleating agent concentration. The maximum effect of the additives occurred at concentrations of 0.3% or greater. (C) 2003 Wiley Periodicals, Inc.