Three samples of ethylene-octadecene copolymers having different quantitative composition were analyzed structurally and in terms of their thermal behavior. The samples were fractionated by temperature rising elution fractionation, presenting different chemical composition distributions (CCD) that are essentially the result of the proportion of incorporated octadecene. The CCD profiles were relatively wide for samples generated by metallocene catalysts. The analyses of the fractions showed that the melting and crystallization temperatures decrease with increasing comonomer incorporation, but this relation is affected by the average molecular weight of the chains. The melting thermograms of those fractions having higher proportions of octadecene may be divided into two characteristic regions: the first one, at a higher temperature, originates from the melting of the least modified chains, which crystallize more perfectly. The second one is formed by the melting of chains having a high degree of comonomer incorporation, which melt in a diffuse manner over a wide range of temperatures, it is probable that the morphology of the crystals formed in this region does not follow the folded chain model, and are better represented by a model involving the alignment of chain segments (bundling).