The secondary crystallization of isotactic polypropylene in its blend with the elastomers ethylene-propylene-diene-terpolymer (EPDM) and trans-polyoctenylene (TOR), which occurs during isothermal crystallization, has been measured by time-dependent recording of X-ray wide-angle scattering. From the results, Avrami exponents were determined, which show that secondary crystallization takes place primarily within the already formed spherulites. Avrami exponents of the primary crystallization have been determined by the same method and also by observation of the spherulitic growth in the polarization microscope. It was found that both elastomers have different effects on the crystallization behaviour of the polypropylene.