An incompletely condensed polyvinylsilses-quioxane (PVSQ) oligomer containing abundant silanol groups was synthesized and characterized by FTIR, H-1-NMR, Si-29-NMR, and MALDI-TOF-MS. Polystyrene/poly-vinylsilsesquioxane (PS/PVSQ) hybrid composites were prepared by an in situ bulk polymerization. The hybrid composites showed higher T-g, T-d, and char yield than PS homopolymer and without mechanical loss. The improvements in the properties of PS/PVSQ hybrid composites can be ascribed to the crosslinking function of PVSQ by silanol condensation in later processing. The hybrids showed different morphology from discrete microstructure to continuous network depending on the concentration of PVSQ. Because of the surface enrichment, a PVSQ protection layer was formed, which made the hybrid surface more hydrophobic. The structure and the reaction mechanism of PS/PVSQ hybrid composites were also investigated. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 2497- 2505, 2010