Hybrid foams, which combine an inorganic polysilsesquioxane network with an organic polystyrene network, were successfully synthesized using high internal phase emulsions (HIPE). Methacryloxypropyltrimethoxysilane (MPS) was copolymerized radically with styrene (S) and divinylbenzene (DVB) in the organic phase of the emulsion. The hydrolytic condensation of the trimethoxysilyl groups formed an inorganic polysilsesquioxane network, which significantly increased the high temperature modulus and thermal stability of the foam. The hybrid foam had an open-cell structure typical of polymer foams based on HIPE (polyHIPE), with a significantly smaller cell diameter than S/DVB polyHIPEs. In the absence of DVB, there is no crosslinked network to restrict the mobility of MPS and the polysilsesquioxane network is more fully developed. At low MPS contents, the thermal stability of the foams is higher for foams with DVB reflecting their higher crosslinking density. At high MPS contents, the thermal stability is higher for foams without DVB, reflecting the more fully developed inorganic network. (C) 2001 Elsevier Science Ltd. All rights reserved.