Five types of melamine-formaldehyde microcapsules, that is, with shells of different compositions [melamine formaldehyde shell or a melamine formaldehydepoly(hexamethylene adipate glycol) shell] and containing or not a flame retardant, diammonium hydrogen phosphate (DAHP), have been prepared by an in situ polymerization method and have been added to an isotactic polypropylene matrix (iPP) by melt blending at 5 wt %. Wide-angle X-ray diffraction and differential scanning calorimetry were employed to investigate the crystallization behavior of the prepared iPP/microparticles composites. The tensile properties and the thermal stability were also evaluated. It was stated that the morphology and the shape of the microparticles not only influence the crystallization behavior but also the thermomechanical properties of these composites. Thus, rougher microparticles act as a nucleating agent for the iPP, and in the presence of microparticles containing DAHP, the alpha- and the beta-crystals are formed. Moreover, the presence of microparticles improves the thermal stability of the iPP. (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2566-2576, 2008