A series of segmented polyurethanes was prepared from the new commercial product KRASOL LBH (linear liquid polybutadiene terminated with secondary hydroxy groups), aromatic diisocyanates, and an aliphatic low-molecular-weight diol. Three types of networks were prepared, with the nature of the crosslinks varied from purely chemical to physical, with a continuous transition to "combined" networks containing both crosslink types. Potassium 2-ethyl hexanoate was used as a catalyst of the in situ formation of trifunctional isocyanurate groups (by cyclotrimerization of isocyanate groups). It was confirmed that mechanical, thermal, and swelling properties are considerably influenced by the ratio of chemical to physical crosslink concentration. The best balance of stressstrain properties was obtained for "combined" networks at a NCO-OH molar ratio of 1.10 when only a few chemical crosslinks are present in predominantly physically crosslinked networks. The presence of thermally stable isocyanurate groups mainly influences the storage shear modulus at elevated temperatures. Small-angle X-ray scattering confirmed the two-phase structure of polyurethanes with a periodicity of 6 -8 nm.