Thermal, thermomechanical, tensile and gas transport properties of aliphatic polycarbonate-based polyurethanes (PC-PUs) and their nanocomposites with bentonite for organic systems were studied. Hard segments are formed from hexamethylene diisocyanate and butane-1,4-diol. All PC-PUs and their nanocomposites feature high degree of the phase separation. Three phase transitions were detected by temperature-modulated differential scanning calorimetry (TMDSC) and dynamic mechanical thermal analysis. TMDSC revealed the filler affinity both to soft and hard segments, even though the affinity to hard segments is much stronger. Elongation-at-break at ambient temperatures is mostly over 700%, which leads together with high tensile strength (in some cases) to very high toughness values (over 200 mJ/mm3). The addition of 1 wt % of bentonite does not practically affect mechanical properties implying its very good incorporation into the PU matrix. Permeabilities and other gas transport properties depend on regularity of PC-diol and on hard segment content, but the variations are insignificant. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2012