The partitioning of the 1-butene co-unit between crystalline and non-crystalline regions of random, homogeneous propylene 1-butene copolymers (PB) has been studied by WAXD, C-13 NMR, and FTIR in a series of copolymers with a concentration of 1-butene ranging from 2 to similar to 20 mol%. A partial inclusion of the 1-butene co-unit in the crystallites is identified by the expansion of the unit cell, and quantified by extracting C-13 NMR spectra of the crystalline regions. For slowly cooled copolymers, about 30% of the chain's 1-butene co-units are incorporated into the crystallites. Analyses of FTIR absorbances associated with crystalline 1-butene provide additional quantitative information on the morphological partitioning of the co-unit and give evidence to support that the incorporation of the comonomer into the crystalline regions is controlled by crystallization kinetics. The presence of the comonomer in the crystalline region affects the observed vibration of the most sensitive iPP 3/1 regularity bands associated with the evolution of crystallites, i.e. 841 cm(-1) (12 isotactic units). The frequency of this band shifts toward higher values with increasing comonomer and with increasing undercooling, in support of an increasing concentration of entrapped crystalline 1-butene. The frequency shift is absent in copolymers with co-units that are excluded from the crystalline regions, such as the 1-octene comonomer. (C) 2011 Elsevier Ltd. All rights reserved.