Cyanate ester (PT-15, Lonza Corp) composites containing the inorganic-organic hybrid polyhedral oligomeric silsesquioxane (POSS) octaaminophenyl(T-8)POSS [1; (C6H4NH2)(8)(SiO1.5)(8)] were synthesized. These PT-15/POSS-1 composites (99/1, 97/3, and 95/5 w/w) were characterized by X-ray diffraction (XRD), transmission election microscopy (TEM), dynamic mechanical thermal analysis, solvent extraction, and Fourier transform infrared. The glass-transition temperatures (T-g's) of the composite with 1 wt % 1 increased sharply versus the neat PT-15, but 3 and 5 wt % 1 in these cyanate ester composites depressed T, All the PT-15/POSS composites exhibited higher storage modulus (E') values (temperature > T-g) than the parent resin, but these values decreased from 1 to 5 wt % POSS. The loss factor peak intensities decreased and their widths broadened upon the incorporation of POSS. XRD, TEM, and IR data were all consistent with the molecular dispersion of 1 due to the chemical bonding of the octaamino POSS-1 macromer into the continuous cyanate ester network phase. The amino groups of 1 reacted with cyanate ester functions at lower temperatures than those at which cyanate ester curing by cyclotrimerization occurred. In contrast to 1, 3-cyanopropylheptacyclopentyl(T-8)POSS [2; (C5H9)(7)(SiO1.5)(8) CH2CH2CH2CN] had low solubility in PT-15 and did not react with the resin below or at the cure temperature. Thus, phase-separated aggregates of 2 were found in samples containing 1-10 wt % 2. Nevertheless, the T-g and E' values (temperature > 285 degrees C) of these composites increased regularly with an increase in 2. (c) 2005 Wiley Periodicals, Inc.