We report the synthesis and characterization of novel elastomeric nanocomposites containing polyhedral oligomeric silsesquioxanes (POSS) as both the cross-linker and filler within a polydimethylsiloxane (PDMS) polymer matrix. These polymer composites were prepared through the reaction of octasilane-POSS (OS-POSS) with vinyl-terminated PI)MS chains using hydrosilylation chemistry. In addition, larger super-POSS cross-linkers, consisting of two pendant hepta(isobutyl)POSS molecules attached to a central octasilane-POSS core, were also used in the fabrication of the PDMS composites. The chemical incorporation of these POSS cross-linkers into the PDMS network was verified by solid-state H-1 magic angle spinning NMR. Based on dynamic mechanical analysis, the PDMS nanocomposites prepared with the octafunctional OS-POSS cross-linker exhibited enhanced mechanical properties relative to polymer systems prepared with the tetrafunctional TDSS cross-linker at equivalent loading levels. The observed improvements in mechanical properties can be attributed to the increased dimensionality of the POSS cross-linker. The PDMS elastomers synthesized from the larger super-POSS molecule showed improved mechanical properties relative to both the TDSS and OS-POSS composites due to the increased volume-fraction of POSS filler in the polymer matrix. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2589-2596, 2009