The thermal stability and crystallization kinetics of isotactic polypropylene (iPP) and iPP/organomont-morillonite (organo-MMT) nanocomposites were investigated with differential scanning calorimetry and thermogravimetry. The incorporation of organo-MMT up to a concentration of 4 wt % did not affect the melting temperature of iPP but did increase the peak thermal degradation temperature by 60degreesC. The isothermal crystallization kinetics showed that the addition of organo-MMT increased the crystallization rate of iPP but reduced the isothermal Avrami exponent. The crystallization temperature of the nanocomposites measured with nonisothermal crystallization was higher than that of plain iPP, and this indicated an enhanced crystallization rate. The nonisothermal Avrami exponent, like the isothermal exponent, decreased with the addition of organo-MMT, and this suggested changes in the crystallite growth geometry. Subsequently, the tensile yield strength and the tensile modulus both increased, but the elongation at break and the notched Izod impact strength did not change significantly. (C) 2003 Wiley Periodicals, Inc.