When subcooled from the melt to any temperature between about 90 to 110 °C, polybutene-1 transforms at a readily measurable rate to a crystalline phase denoted "form 2," which is typically spherulitic. The kinetics of this process have been measured by optical and dilatometric techniques. The results are consistent with the concepts that the initiation is heterogeneous, that the initiation is followed by spherical growth (Avrami n=3), and that the growth rate is nucleation controlled. Values arc obtained for the surface free energies and related quantities that are consistent with crystallization by chain folding: σ=7.2 erg/cm2 or 7.2 mJ/m2, σ e =15.5 erg/cm2 or 15.5 mJ/m2, and work of chain folding q=1.7 × 10-13 erg/fold. (For comparison, q is 2.1 × 10-13 erg/fold for polyethylene.) The work of chain folding of polybutene-1 is also compared with that of polychlorotrifluoroethylene, and it is shown that an increase of q is connected with an increase of equilibrium melting temperature. Crystal form 2 subsequently converts slowly near room temperature to a crystalline phase denoted "form 1" of different helicity and density, and the rate of this crystal-crystal transformation is also studied. The Avrami parameter for the form 2→form 1 process is n = 2. The process appears to be nucleation controlled. It is shown that form 1 is, everywhere below its melting point, the stable form. The equilibrium melting temperature for form 2 is estimated to be about 128 °C, and for form 1 about 141 °C. A degree of crystallinity scale based on specific volume is established. The degree of crystallinity of form 2 is about 52 percent, and after conversion to form 1, the crystallinity is about 77 percent.