Powder diffraction measurements on five unoriented sample-, of poly (ethylene-co-hexe n e) were performed in the temperature range 23-200 degrees C using synchrotron X-radiation in combination with an area detector. Melting and recrystallization was found to improve the crystallinity of the samples, resulting in a denser packing along [010] or b for the most crystalline samples. A high content of cohexene (branching) reduces both the melting temperature T-m and the crystallinity. There appears to be a slight increase in T-m with increasing molecular weight M. of the sample. The coefficient of thermal expansion (CTE) along [100], alpha(a), was found to be always positive, in the range (26-34) x 10(-5) K-1 up to melting, with the larger values for the most crystalline samples. The CTE in the chain direction, i.e. alpha(c) along [001], is negative, ranging from (-0.6 to -8.0) x 10(-5) K-1. The thermal response in the [010] direction is more complex, in most cases being significantly different in the heating and cooling sequences. The unit cells expand nearly linearly in the temperature range from RT to about 20 degrees C below T-m. Increasing T towards T-m brings about an enhanced asymmetry in the C-C-H valency angles and a small rotation of the polymer chains with a concomitant expansion of the interchain contacts lying approximately in the bc-plane. Melting and recrystallization induce a shortening of these contacts and both the atomic and the molecular movements involved in the structural changes are reversed during cooling. (c) 2007 Elsevier Ltd. All rights reserved.