A layer multiplying coextrusion process was used to produce multilayered polypropylene/polystyrene (PP/PS) films with various nucleating agents. When heated into the melt, the thin PP layers broke up into submicron PP droplets that exhibited fractionated crystallization. If the initial PP layers were 20 nm or less, the resulting droplets exhibited exclusively homogeneous nucleation. If a nucleating agent was added, the systematic departure from homogeneous nucleation provided insight into the nature of the heterogeneous nucleation. In this study, we used thermal analysis, atomic force microscopy (AFM), and wide angle X-Ray scattering (WAXS) to examine the effect of two nucleating agents. We confirmed with WAXS and AFM that a soluble sorbitol nucleating agent for the PP alpha-form operates in three concentration regimes as proposed in a previous study. Morphologically, homogeneous nucleation of the submicron droplets produced a granular texture. The correlation length from small-angle X-Ray scattering (SAXS) suggested that the grains contained 1-3 mesophase domains. Drawing on classical nucleation theory, the critical size nucleus of an individual mesophase domain was estimated to be about 2 nm(3), which was considerably smaller than the mesophase domain. This pointed to mesophase crystallization that included the processes of nucleation and growth. Additional experiments were performed with nucleating agents for the PP beta-form. However, they were not effective in nucleating crystallization of the droplets, presumably because they were essentially insoluble in PP and the nucleating particles were too large to be accommodated in the PP droplets. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 159-171, 2011