Binary mixtures of 1,3: 2,4-di(3,4-dimethylbenzylidene) sorbitol (DMDBS) within the melt of polypropylene ( PP) were studied at DMDBS contents of 0.4 and 1.0 wt %. DMDBS serves as a nucleating agent in PP crystallization by formation of a nanofibrillar network. The kinetics of the DMDBS solidification process within the PP melt and the ensuing nanofibrillar structure were studied by in situ small-angle X-ray scattering (SAXS) analysis combined with imaging by electron microscopy. The dynamic lag of the fibrillar structure formation kinetics and its temperature dependence indicate a nucleation and growth mechanism, controlled by the rate of nucleation. Investigation of the fibrillar structure by electron microscopy indicates a complex structure in which long and thin fibrils ( less than 100 nm in cross-section) are composed of thinner nanofibrils (less than 10 nm in cross-section).