DSC, SAXS/WAXS and SANS have been used to study the thermal behaviour of a model polyurethane elastomer. Several poly(ether-urethane)s were synthesized, all of which contained the same fraction of MDI/BDO hard segment (50%), but which contained di †erent PPO soft segments : I contained only hydrogenous soft segments, II was a contrast-matched material containing 31/ 69 wt.% D/H soft segments, and III was a polymer containing only deuterated soft segments. The SANS patterns obtained from I and III showed the microdomains to be of similar size and also showed good agreement with the (140 Ó) SAXS pattern obtained from I, conÐrming that it is the microdomains that are the scattering entities in both techniques. The contrast-matched material II was heated in order to investigate changes in the conformation of the soft segments with temperature. These changes were correlated with changes taking place in the long period, obtained from SAXS experiments on the hydrogenous material. It was found that at low temperatures the soft segments were in extended conformations as a result of the stresses imposed on them during the crystallization (and microphase separation) of the hard segments. On heating to 100È140¡C, the radius of gyration of the soft segments decreased, indicating that the soft segments were relaxing to take the form of random coils. SAXS showed the long period to increase steadily in this temperature range, which was attributed to the preferential melting of smaller crystalline domains. At P180¡C, SANS showed the radius of gyration of the soft segments to increase as phase mixing took place, much as a homopolymer is swollen by a good solvent.