The intermolecular structural heterogeneity of a propylene-ethylene random copolymer was studied by preparative temperature rising elution fractionation combined with GPC, C-13-NMR, differential scanning calorimetry, and wide-angle X-ray diffraction analysis of the obtained fractions. The isotacticity of fractions increased with increasing elution temperature, and the ethylene content decreased monotonously. Fitting of the obtained comonomer triad sequences by Bernoullian and first-order Markovian statistical models indicated that lower isospecific active sites are more active toward ethylene. The isolated ethylene unit disrupted the crystallizable isotactic sequence and lowered the crystallizability of the polypropylene chain. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 232-239, 2005.