A small- and wide-angle X-ray scattering study was performed on two metallocene catalyzed isotactic polypropylene (miPP) resins. The results were compared with two similar molecular weight Ziegler-Natta catalyzed isotactic polypropylene (zniPP) materials. Wide-angle X-ray diffraction (WAXD) results showed the existence of two crystalline structures in the metallocene samples, the alpha-monoclinic and gamma-orthorhombic crystal structure, with increasing relative amounts of gamma-orthorhombic phase as the lamellae thickness increased. Differential scanning calorimetry (DSC) scans exhibited a melting peak for each crystal structure. The metallocene resins had the same equilibrium melting temperature (186 +/- 2 degrees C) as the high tacticity Ziegler-Natta (ZNHT) resin, whereas a second Ziegler-Natta resin had a lower equilibrium melting temperature (178 +/- 2 degrees C). The equilibrium melting temperature for the gamma-orthorhombic crystal structure in the metallocene resins was found to be 178 +/- 4 degrees C. The results were explained by the distribution of defects within the miPP chains, generating higher fold surface free energies for the miPP resins.