A series of polyesterurethanes with differing block length and constant composition have been synthesized for rheological studies. Hard segments based on isophorone diisocyanate and 1,4-butanediol and soft segments based on polycaprolactone to ensure high thermal stability and to prevent high melting point crystallinity enabled long-duration rheological characterization at high temperatures. DSC and SAXS revealed that, at any fixed temperature above the polyester melting point, the degree of microphase separation increased with block length, with the shortest block lengths being almost single-phase. Temperature-resolved SAXS experiments demonstrated gradual microphase mixing of the microphase-separated materials as the temperature increased. In addition, the SAXS data for one material were shown to obey the predictions of the mean field theory, allowing a mean field estimate of the spinodal temperature to be calculated. Frequency sweep dynamic mechanical experiments show viscoelastic behavior characteristic of a homopolymer for all materials at high temperatures, and master curves can been constructed using the principle of time-temperature superposition. A failure of time-temperature superposition was observed at lower temperatures in materials with large block length. Temperature-resolved SAXS studies suggest that this failure is related tb the onset of microphase separation in these materials at low temperatures. In the high-temperature regime, where master curves can be constructed, the WLF equation with universal parameters fits the experimental shift factors very well if an apparent single-phase glass transition temperature (T-g) is used. In addition, the relaxation time and the Newtonian viscosity of the polyurethanes show a strong dependence on block length, with a power law exponent of about 5.