Crystallization kinetics and crystallinity development of PA6 droplets having sizes from 0.1 to 20 mu m dispersed in immiscible uncompatibilized PS/PA6 and reactively compatibilized (PS/Styrene-maleic anhydride copolymer= SMA2)/PA6 blends are reported. These blend systems show fractionated crystallization, leading to several separate crystallization events at different lowered temperatures. Isothermal DSC experiments show that micrometer-sized PA6 droplets crystallizing in an intermediate temperature range (T-c similar to 175 degrees C) below the bulk crystallization show a different dependency on cooling rate compared to bulk crystallization, and an athermal crystallization mechanism is suggested for PA6 in this crystallization temperature region. The crystallinity in these blends decreases with PA6 droplet size. Random nucleation, characteristic for a homogeneous nucleation process, is found for sub-micrometer sized PA6 droplets crystallizing between T-c 85 and 110 degrees C using isothermal DSC experiments. However, crystallization in the PA6 droplets is most likely initiated at the PA6-PS interface due to vitrification of the PS matrix during crystallization. Very imperfect PA6 crystals are formed in this low temperature crystallization region, leading to a strongly reduced crystallinity. These crystals show strong reorganization effects upon heating. (c) 2005 Elsevier Ltd. All rights reserved.