Calorimetric experiments at cooling rates comparable to those during injection molding, as an example, are needed to study phase transitions under conditions relevant for processing. Ultra fast scanning calorimetry is a technique which provides a means to analyze the materials of interest under rapid cooling conditions and it is a promising technique by which the crystallization behavior of composite systems based on fast crystallizing polymers like isotactic polypropylene (iPP) can be studied. By combining conventional DSC and ultra fast chip calorimetry isothermal crystallization experiments were performed in the whole temperature range between glass transition and melting temperature of iPP. Because of the very small time constant of the calorimeter, isothermal crystallization processes with peak times down to 100 ms were investigated after cooling the sample from the melt at 2000 K/s. iPP grafted with maleic anhydride (PPgMA) -montmorillonite clay nanocomposites were studied. The influence of various clay loadings on the crystallization behavior of PPgMA at different temperatures was followed by ultra fast isothermal calorimetry. PPgMA clay nanocomposites showed a variation in crystallization peak times with different clay loadings at crystallization temperatures between 70 degrees C and 100 degrees C. No influence of clay loading was observed at lower crystallization temperatures. At these temperatures, where the mesophase is formed and homogeneous nucleation is expected, the contribution of the clay as a nucleating agent is negligible. For crystallization at about 80 degrees C, where the alpha-phase is formed, the nucleating effect of the clay is observed yielding complex crystallization kinetics. In the temperature range 75-85 degrees C in some nanocomposites a double peak during isothermal crystallization was observed corresponding to a fast and a slow crystallization processes occurring simultaneously. At higher temperatures, above 120 degrees C, the clay slightly retards the crystallization process. (C) 2007 Elsevier Ltd. All rights reserved.