Polyhedral oligomeric silsesquioxanes (POSS) epoxy hybrid composites have attracted much research interest because of their unique structure, versatile synthetic approaches, and changeable properties through molecular tailoring. Octakis(dimethylsiloxypropylglycidyl ether)silsesquioxane and octakis(dimethylsiloxyethylcyclohexenyl epoxide)silsesquioxane were synthesized and cured with 4,4'-methylenebis(cyclohexylamine) and 4-methylhexahydrophthalic anhydride to prepare the highly crosslinked hybrid materials. The thermochemical data from DSC analysis and FTIR show that the curing reactions of POSS epoxy are more difficult than diglycidyl ether of bisphenol A (DGEBA) resin because of steric hindrance. The multifunctional structure of POSS can form highly crosslinked network throughout the composites, therefore the polymer main framework is frozen and cannot move freely. Some POSS composites do not show glass transitions and the observed relaxation for other POSS composites is likely due to the motion of tethers between POSS cores. The moduli of POSS composites, which decrease slowly with temperature increasing, are much higher than that of DGEBA resins at high temperatures. Although the coefficients of thermal expansion of POSS composites are larger than that of DGEBA resins at low temperatures, they are less dependent on temperature and relatively low at high temperatures. The unique thermal and mechanical properties of POSS composites make them potential candidates for applications in high temperature and temperature variable environments. (c) 2007 Wiley Periodicals, Inc.