Segmented polyurethane block copolymers were synthesized using 4,4'-methylenediphenyl diisocyanate (MDI) and 1,4-butanediol (BDO) as hard segments and various soft segments derived from poly(hexamethylene oxide) (PHMO) and poly(dimethylsiloxane) (PDMS)-based macrodiols and mixtures thereof. The microstructure and degrees of phase separation were characterized using a variety of experimental methods. Copolymers synthesized with the PDMS macrodiol and from PDMS/PHMO macrodiol mixtures were found to consist of three phases: a PDMS phase: hard domains; and a mixed phase of PHMO, PDMS ether end group segments and some dissolved hard segments. Two models were used to characterize the small-angle X-ray scattering from these copolymers: pseudo two-phase and core-shell models. Analysis using both methods demonstrates that as the PDMS content in the soft segment mixture increases, the greater the fraction of hard segments involved in hard domains than are dissolved in the mixed phase. Findings from analysis of the carbonyl region of FTIR spectra are also in agreement with greater hard/soft segment demixing in copolymers containing higher PDMS contents. (C) 2009 Elsevier Ltd. All rights reserved.