1,3:2,4-Di 3,4-dimethylbenzylidene) sorbitol (DMDBS) is a known nucleator and clarifier for polypropylene (PP). It is also known that in the useful concentration range of roughly 0.2-2%, it forms nanofibrillar structures within the PP melt. The kinetics of the DMDBS solidification process within the PP melt and formation of the ensuing nanofibrillar structure are studied by in-situ small angle X-ray scattering (SAXS) analysis. 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. Examination of the polystyrene (PS)/DMDBS system indicate structure formation only in compounds containing 2.5% DMDBS, as evident in the rheological behavior. Light microscopy reveals that the fibrillar cross-section in this case is larger by an order-of-magnitude in comparison to the structure in PP.