C-13 nuclear magnetic resonance (n.m.r.) and Fourier transform infra-red (FTi.r.) spectroscopies, as well as wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (d.s.c.) and temperature rising elution fractionation (TREF), have been combined to measure the composional heterogeneity of a commercial propylene-ethylene block copolymer. It has been shown that the copolymer contains molecular species with a wide variation in composition, and the copolymer products range from amorphous ethylene-propylene rubbers (EPR) to crystallisable propylene-ethylene statistical copolymers, polyethylene and polypropylene homopolymers as well as blocks of various lengths. The so-called block copolymer was composed of about 15% amorphous EPR, 5% random copolymer, 28% block copolymers with long propylene and long ethylene sequences, and 52% homopolypropylene. The crystallisation and melting behaviour of these fractions have been investigated.