The organic-inorganic hybrids involving epoxy resin and polyhedral oligomeric silsesquioxanes (POSS) were prepared via in situ polymerization of diglycidyl ether of bisphenol A (DGEBA) and 4,4'-diaminodiphenylmethane (DDM) in the presence of the two structurally similar POSS monomers. The organic groups on silsesquioxane cage are aminophenyl and nitrophenyl groups, respectively. The curing reactions were started from the initially homogeneous mixture of DGEBA, DDM and the POSS cages. The inorganic-organic hybrids containing up to 20 wt% of POSS were obtained. The morphologies of the resulting hybrids were quite dependent on the types of R groups in the POSS monomers. The phase separation induced by polymerization occurred in the hybrids containing octanitrophenyl POSS (OnpPOSS) and the spherical particles of POSS-rich phase (<0.5 mum in diameter) were uniformly dispersed the continuous epoxy matrix as shown by scanning electronic microscopy. In marked contrast to the OnpPOSS-containing hybrids, the octaaminophenyl POSS (OapPOSS)-containing nanocomposites exhibited a homogeneous morphology. Differential scanning calorimetry and dynamic mechanical analysis showed that the glass transition temperatures (T-g) of the POSS-containing hybrids were lower than that of the control epoxy. The moduli of glass states for the hybrids are significantly higher than that of the control epoxy. For the OapPOSS epoxy nanocomposites the storage moduli of the rubbery plateau were higher than that of the control epoxy when the contents of POSS are less than 20 wt%, indicating the nanoreinforcement effect of POSS cages. Thermogravimetric analysis indicates that the thermal stability of the polymer matrix was not much sacrificed by introducing a small amount of POSS, whereas the properties of oxidation resistance of the materials were significantly enhanced. The OapPOSS epoxy nanocomposites displayed more pronounced improvement than the OnpPOSS hybrids, which could be ascribed to the nanoscaled dispersion of POSS cages and the formation of tether structure of POSS cages with epoxy matrix. (C) 2004 Elsevier Ltd. All rights reserved.