Abstract The isothermal crystallization and phase II → I transformation of bulk polybutene‐1 at various temperatures have been characterized by x‐ray diffraction. The crystallization rate can be increased or decreased by the addition of various solids, with sodium salicylate and talc, e.g., increasing the rate by >100% and, unexpectedly, decreasing the rate by >300%, respectively. The nucleating agents have no effect on the transformation rate, which, however, is increased slightly in the presence of water and ethanol and significantly in the presence of liquids that are solvents at elevated temperatures or by temperature cycling. As shown previously, the transformation rate of nontreated polymer is greatest near 25°C, decreasing at higher and lower temperatures and is increased by tension or uniaxial compression.