Model segmented polyurethanes (SPUs) prepared from either oxypropylene glycol oligomer or butylene adipate glycol oligomer, both of molar mass 2 kg/mol (soft fragments, SFT), and three different diisocyanates (all-trans 4.4＇-dicyclohexylmethane diisocyanate, t, t-HMDI-1.0; HMDI with 20% of trans isomers, t, t-HMDI-0.2; and 4, 4＇-diphenylmethane diisocyanate, MDI) (stiff fragments, STF) were characterized by specific heat capacity measurements in the temperature interval 140-540 K, and by wide-angle and small-angle X-ray scattering at room temperature. Limited miscibility of SFT and STF chain components resulted in incomplete separation into a regular three-dimensional macrolattice of STF-rich microdomains and SFT-rich microphases. The composition of STF-rich microdomains was estimated by fitting the softening temperatures to the Couchman＇s equation, whereas the relative contents of SFT-rich and STF-rich microphases were assessed by comparing the specific heat capacity change at the glass transition temperatures to corresponding additive values. The overall degree of microphase separation, as well as the mean macrolattice spacings between STF microdomains decreased in the order, MDI > t, t-HMDI-1.0 much greater than t, t-HMDI-0.2. The conformation of STF fragments within the STF-rich microdomains changed h-om nearly extended (for MDI) through slightly contracted (for t, t-HMDI-1.0) to strongly contracted (for t, t-HMDI-0.2).