In this paper the relation between the blend phase morphology and the fractionated crystallization behavior of PA6 in reactively compatibilized immiscible PS/PA6 and (PPE/PS)/PA6 immiscible blends is studied. Reactive compatibilization is used as an effective tool for controlling the blend phase morphology, and to reduce the PA6 dispersed droplet size. As reactive compatibilizers, SMA2 and SMA17 are used, which differ in their level of miscibility with the amorphous PS and (PPE/PS) components. With SMA2 a strong shift of PA6 crystallization to much higher supercoolings than before is found after compatibilization resulting in crystallization at temperatures as low as 85 degreesC. This is ascribed to the strong decrease of the droplet sizes down to 100-150 nm. Nucleation experiments show that heterogeneous bulk nucleation can be reintroduced in the submicron-sized PA6 droplets by adding enough nucleating agents of sufficient small size. The degree of fractionated crystallization is found to depend on the interface between PA6 droplets and surrounding medium, as it is influenced by vitrification of the matrix polymer and by the location of the compatibilizers SMA2 and SMA17 The method used for mixing the reactive compatibilizer with the blend components also affects the fractionated crystallization process. (C) 2004 Elsevier Ltd. All rights reserved.