We report results from time-resolved small/wide angle X-ray scattering of well-characterized isotactic polypropylene (iPP), in its pure form as well as loaded with nucleating additives of different shapes (isotropic, elongated, platelet-like). Isothermal crystallization of these systems has been studied both under quiescent conditions and after application of a shear pulse. A quantitative analysis is presented that couples the applied shear field and the specific additive to the nucleation of iPP and the subsequent kinetics of crystallization. The effect of flow is only relevant if its contribution to the nucleation density is at least of the same order of magnitude as the nucleation density under quiescent conditions. Anisotropic additives show strong interactions with the flow field leading to extra nucleating sites. (c) 2008 Elsevier Ltd. All rights reserved.