Reliable isothermal crystallization kinetic studies can be achieved by differential scanning calorimetric techniques only under restricted conditions. In the case of isotactic polypropylene, our results indicate that those conditions are met in the range of 3degreesC below the isothermal crystallization temperature T-c. Experimentally, it is only in this range when the complete crystallization peak can be registered by the DSC and depicted in a thermogram. Just around this temperature interval, the Avrami exponent n = 3 for bulk crystallization, whereas for any other test the isothermal temperature T-it, nonisothermal conditions prevail and the Avrami exponent deviates from the expected value. (C) 2004 Wiley Periodicals, Inc.