Novel polyurethane (PU)/polyhedral oligomeric silsesquioxane (POSS) hybrid elastomers were synthesised via reaction carried out in bulk. The isocyanate octasubstituted POSS (NCOPOSS) was used as the crosslinking agent for the polyurethane systems, which were similar to those commercially available for the production of urethane elastic materials (in contrast to model systems previously reported in literature). The structure and amorphous morphology of the resulting elastomers were confirmed by FTIR ATR spectroscopy and WAXS measurements, respectively. The elastomers were characterized by their mechanical (tensile, hardness) and thermal (T-g, thermal stability) properties. It was found that the incorporation of small amount (1.2-2.5 wt.%) of the NCOPOSS into the formulation resulted in an improvement or deterioration in mechanical properties of the polymer, depending on the diisocyanate used. Further increase in the NCOPOSS concentration resulted only in a loss of mechanical properties. A possible explanation is an over-crosslinking or wastage of PUSS-deriving NCO groups (which did not react earlier with bulky polyols) in the reaction with the chain extender leading to cycle formation. Incorporation of the NCOPOSS into the PU elastomer reduced somewhat thermal stability of the latter probably due to the poor thermal stability of the NCOPOSS itself. (c) 2012 Elsevier Ltd. All rights reserved.