Urethane elastomers having hard segments based on CHDI [trans-1,4-diisocyanatocyclohexane] and BD [1,4-butanediol] are notable for their high softening temperatures and low hysteretic heat buildup. Experimental studies as a function of temperature using both wide- and small-angle X-ray diffraction of a series of such elastomers, incorporating PTMO [poly(tetramethylene oxide)] soft segments, reveal the persistence of hard-segment microdomains up to 275 degrees C. These hard-segment microdomains appear to be paracrystalline at ambient temperature, transforming to a glassy structure above ca. 100 degrees C. The glassy hard-segment structure is rigid to at least 275 degrees C, conferring rubbery properties on the material. Electron density variance data obtained by SAXS indicate that there is little hard- and soft-segment mixing at 30 degrees C for elastomers for which the molar ratio BD/PTMO is less than 2.0, with some segmental mixing indicated for higher BD/PTMO ratios. Changes in the microdomain structure with temperature are, for the most part, reversible for an elastomer with shorter hard segments corresponding to BD/PTMO = 1.5 but show marked irreversibility for one in which BD/PTMO = 3. These results are found to be consistent with models based on model compound data for the hard-segment packing.