The distribution of the co-units between the crystalline and amorphous phases in a series of five ethylene-1-butene copolymers has been studied by means of X-ray diffraction measurements in the framework of Balta-Calleja’s model for the crystalline insertion of structural chain defects. This model assumes that the side groups are accommodated at interstitial positions within the crystalline phase owing to 2g1 kinks of the main chain. The defect concentration in the crystalline phase of solution-crystallized samples is shown to be fairly insensitive to the solution concentration. Besides, the determination of the crystalline and amorphous chain defect concentrations in the melt-crystallized copolymers, together with the study of their melting behaviour, allowed us to draw the ethylene-rich side of the poly(ethylene-co-butene) phase diagram for normal pressure crystallization. The self-consistency of the present data and the favourable comparison with recent C-13 n.m.r. spectroscopic measurements give support to the interstitial kink model. In addition, our results clearly indicate that the solubility of the butene co-units in the crystal is governed by thermodynamic equilibrium.