Properties of cycloolefin copolymer (COC)-polyhedral oligomeric silsesquioxanes (POSS) nanocomposites, prepared by melt compounding, have been investigated. Composites retained the optical transparency of the matrix up to a nanofiller content of 5 wt %. whereas for higher percentages the formation of a crystalline phase, due to the presence of agglomerated nanoparticles, was revealed by X-Ray diffraction (XRD). Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) evidenced a significant decrease of the glass transition temperature (T,) and of the thermal decomposition rate, with the POSS content. Similarly, at increasing filler content, both quasi-static tensile tests in the solid state and rheological measurements on melts showed a decrease of measured parameters (elastic modulus, ultimate elongation, loss and storage shear modulus, viscosity). These results were attributed to the formation of a soft interphase at the nanoparticles /polymer boundary due to the presence of isobutylic groups on POSS nanoparticles that limited the stress transfer process and acted in the COC matrix as a molecular lubricant agent. A small increase of the receding contact angle with the nanofiller content was also detected from dynamic contact angle (DCA) measurements. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 2270-2279,2009