Morphology development during isothermal crystallization in equal molecular weight isotactic polypropylene (iPP), syndiotactic polypropylene (sPP), and iPP/sPP blends was studied with time-resolved Simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) with synchrotron radiation. The sPP melting point is 15-20 degreesC below that of the iPP component, and sPP multiple melting is not affected by blending for 50-100 wt % sPP compositions. SAXS and WAXD (at 115 and 137.5 degreesC) show that sPP crystallizes more slowly than iPP. The sPP long spacing is larger than that of iPP at both crystallization temperatures, exhibits a broader distribution, and changes to a greater extent during crystallization. Differential scanning calorimetry (DSC) cooling and SAXS/WAXD measurements show iPP crystallizing first and nearly to completion before sPP in a 50:50 iPP/sPP blend. At 115 degreesC, iPP crystals nucleate sPP in a 50:50 blend and modify the sPP lamellar spacing. The nucleation does not overcome the large difference in the iPP and sPP rates at 137.5 degreesC. Before sPP crystallization in a 50:50 blend (115 degreesC), the iPP long spacing is not affected by molten sPP. The iPP long spacing is slightly expanded by molten sPP, and the WAXD induction time is delayed at 137.5 degreesC. The observed iPP long spacing in the presence of molten sPP is consistent with previously reported results for iPP/atactic polypropylene (aPP) blends of similar molecular weight. Quantitative differences between the two types of blends are consistent with previously reported thermodynamic rankings. ( C) 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1876-1888, 2001.