A kinetic model has been developed to describe the behavior of semicrystalline polymer interfaces. In these systems, the competition between interdiffusion and crystallization drives the overall transport. The crystallization rate is based on the Avrarmi equation, in which the Avrami exponent and the crystallization rate constant as a function of temperature and blend composition are determined by in situ optical microscopy and differential scanning calorimetry. The mutual diffusion coefficient is obtained by using the fast mode theory. The predicted density profiles as a function of position are used to extract the interfacial widths, which are affected by molecular weight, equilibrium degree of crystallinity, and temperature. At low temperatures, the crystallization rate is fast, and the crystals present near the interface hinder the interdiffusion, causing the interfacial width to be small. At high temperatures, the crystallization is much slower, and interdiffusion dominates crystallization. The model predictions are compared with interfacial behavior observed by transmission electron microscopy and small angle X-ray scattering in a system composed of polyethylene and isotactic polypropylene and the agreement with the experiments is satisfactory. (c) 2005 Elsevier Ltd. All rights reserved.