The origins of multiple endotherms in segmented polyurethane block copolymers are investigated by simultaneous small-angle X-ray scattering (SAXS) and differential scanning calorimeter (DSC) analyses. The materials examined contain hard segments composed of a 4,4’-methylenediphenyl diisocyanate chain extended with butanediol and soft segments of oxyethylene end-capped poly(oxypropylene). Two distinct endotherms are observed in all materials. A high-temperature endotherm above 200 °C is attributed to melting of microcrystalline hard segments. A lower temperature endotherm is assigned to the onset of microdomain mixing of “noncrystalline” hard and soft microphases that accompanies the microphase separation transition from an ordered to disordered phase. The two endotherm temperatures are found to depend on the annealing temperatures and indirectly on the apparent crystallinity. This behavior is discussed in terms of temperature-induced changes in the hard microdomain structure. © 1986, American Chemical Society. All rights reserved.