Polyurethane-ureas with dibenzo-18-crown-6 in the rigid chain fragments were characterized by small-angle X-ray scattering (SAXS), differential scanning calorimetry (d.s.c.), deformation calorimetry and gas transport measurements. As-cast samples are structurally microheterogeneous systems in which the composition and the relative content of different microphases varies with the chain length and the chemical nature of the soft component. As a general trend, the thermodynamic affinity between rigid and soft fragments becomes worse (i.e. the degree of microphase separation alpha* increases) when the chain length of the latter increases and/or when an oligoether component is replaced with an oligoester of similar length. Thermoelastic and gas transport properties of the samples depend not only on their nominal chemical composition and alpha* but also on the pattern of molecular packing within the rigid fragment-containing microphases.