The thermal behaviour and morphology of a series of homogeneous ethylene-1-octene copolymers, covering a wide range of 1-octene content up to 44 mole%, is studied using differential scanning calorimetry (DSC), time-resolved small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD). The influence of the comonomer (1-octene) content, as well as the influence of the comonomer type (branch length) are investigated by comparison with previous results on homogeneous ethylene-propylene and ethylene-l-butene copolymers. Special emphasis is put on copolymers with high comonomer contents or low densities (<870 kg/m(3)) at room temperature. The thermal behaviour and the morphology of the present copolymers reveal no discontinuities with increasing comonomer content and support a model in which the morphology changes gradually from a lamellar base morphology into a granular one consisting of small blocky structures and, possibly via fringed-micelles, into a morphology consisting of loosely packed ethylene sequences. The latter structures are too small and/or too imperfect to be detected by WAXD. However, SAXS and DSC are sensitive to them and are useful techniques over the whole comonomer content range. (C) 2000 Published by Elsevier Science Ltd. All rights reserved.