This work investigated the thermal behavior of a series of thermoplastic poly(ether-urethane)s containing 36-71% by weight hard segments derived from 4,4'-methylene-bisphenylisocyanate and butane-1,4-diol, with poly(tetramethylene oxide) soft segments. In all materials studied, differential scanning calorimetry revealed the presence of a T-1 endotherm similar to 20-30 degrees C above the annealing temperature. Morphological changes during heating were observed using small-angle X-ray scattering; the data was analyzed using "globular" models based on a one-dimensional statistical lattice or the Percus-Yevick description of liquids, both of which appeared to provide good descriptions of these materials. The results indicated that the T-1 endotherm coincided with the onset of morphological changes during heating. Possible explanations are discussed, based on the melting of small hard-segment crystals or an activation energy associated with transient segmental mixing. (c) 2006 Wiley Periodicals, Inc.