The molecular structure of three isotactic polypropylenes polymerized using either slurry or bulk process and heterogeneous Ziegler-Natta catalysts without external donors was studied in this work. The influence of external donor addition was also examined. The samples were fractionated according to stereoregularity using a solvent gradient extraction method, and calorimetric properties, C-13 nuclear magnetic resonance (n.m.r.) pentad tacticities and molecular weights of the fractions were determined. Statistical analysis was also applied to the C-13 n.m.r. results. The samples polymerized with the novel catalyst system had the conventional stereostructure arising from a catalyst-site-controlled propagation mechanism. When compared with commercial polypropylenes, the amount of clustered defects was increased in the products made with the novel catalyst system. The addition of an external donor diminished the amount of highly atactic material, but the isotacticity of long-chain material was also increased. Qualitatively similar defects were detected in all samples, only the amount of defects varied depending on the catalyst structure and polymerization process used. Surprisingly, the bulk polymerization process seemed to favour the formation of non-isotactic material.