The kinetics of crystallization of isotactic polybutene-1 has been investigated as a function of the concentration of propylene chain defects between 0 and 20 mol% by temperature/time-resolved wide-angle X-ray scattering during slow cooling of the melt followed by annealing at ambient temperature. Slow cooling of the melt of the homopolymer leads to formation of Form II mesophase which then transforms on isothermal annealing at ambient temperature to Form I crystals within a period of 10-20 days. Presence of propylene co-units/chain defects in polybutene-1 leads to a retardation of Form II mesophase formation on cooling and simultaneously an acceleration of the mesophase crystal phase transformation. In random butene-1/propylene copolymers containing about 7 mol% propylene crystallization is completed in less than a day. For copolymers with more than 7 mol% propylene, Form I crystals develop by both reorganization of Form II mesophase and directly from the melt. The study is completed by evaluation of the effect of copolymerization on the crystallinity and crystal perfection. (C) 2013 Elsevier Ltd. All rights reserved.