Novel silsesquioxane-cross-linked porous nanocomposites with densities of around 1.5 x 10(2) kg/ m(3) were synthesized within high internal phase emulsions (HIPE) in which 2-ethylhexyl acrylate (EHA) was cross-linked and reinforced with vinyl silsesquioxane (VSQ). The molecular structure, porous structure, thermal properties, mechanical properties, and pyrolysis were investigated. The tan delta peak temperature, tan delta full width at half-maximum, room-temperature modulus, and stress at, 40% strain all increased in a linear fashion with increasing VSQ content, reflecting the increase in cross-linking through reaction with the reinforcing Si-O network. The VSQ-containing polyHIPE were compared to polyHIPE based on EHA cross-linked with divinylbenzene (DVB) and reinforced by reaction with a polyhedral oligomeric silsesquioxane (POSS) bearing one vinyl group. On a molar basis, cross-linking with DVB produced higher tan delta peak temperatures and moduli, and this was related to the presence of unreacted VSQ vinyl groups. However, the POSS-containing polyHIPE had significantly higher tan delta peak temperatures and moduli for similar Si contents. Porous inorganic monoliths were produced on pyrolysis of these VSQ-containing polyHIPE.