A series of copolymers and blends based on polybutylene were characterized using13C NMR and thermal analysis. Optical microscopy techniques were used to measure crystallization rates and examine their morphology, and this behavior was related to their composition. As expected, the highest melting and crystallization temperatures, and the fastest crystallization rates were obtained for the polybutylene homopolymer. Copolymerized ethylene randomly distributed along the chain, even at a level of 1%, significantly reduced the melting temperature and crystallization rate. Increasing the ethylene content to 5% results in a polybutylene which crystallizes with difficulty to produce poorly defined spherulites. Four blends based on a polybutylene containing 1% ethylene were studied. A dispersed crystalline phase could be detected in all cases. Two of the blends contained 6-7% ethylene but differed in molar mass. Their melting and crystallization behavior was similar although the spherulite growth rate for the lower molar mass material was a little higher. A second pair of blends contained 5-6% polypropylene and included a “nucleated” and a “nonnucleated” grade. Results for these suggested that the polypropylene itself was acting as a nucleating agent.