Immiscible blends of 80 wt% polypropylene (PP) with 20 wt% polyamide (PA) or polycarbonate (PC) were prepared by melt mixing with or without the addition of 5% nanosilica. The nanosilica produced a strong reduction of the disperse phase droplet size, because of its preferential placement at the interface, as demonstrated by TEM. Polarized Light Optical Microscopy (PLOM) showed that adding PA, PC or combinations of PA-SiO2 or PC-SiO2 affected the nucleation density of PP. PA droplets can nucleate PP under isothermal conditions producing a higher nucleation density than the addition of PC or PC-SiO2. PLOM was found to be more sensitive to determine differences in nucleation than non-isothermal DSC. PP developed spherulites, whose growth was unaffected by blending, while its overall isothermal crystallization kinetics was strongly influenced by nucleation effects caused by blending. Addition of nanosilica resulted in an enhancement of the strain at break of PP-PC blends whereas it was observed to weaken PP-PA blends. (c) 2013 Elsevier Ltd. All rights reserved.