Epoxy resinsilica nanocomposites with spherical silica domains with 29.0 nm in diameter in an epoxy resin matrix were synthesized from Bisphenol-A type epoxide monomer, 2,2-bis(4-glycidyloxyphenyl)propane (DGEBA), and perhydropolysilazane (PHPS, -[Si-2-NH](n)(-)). The volume fraction of silica domain in the composite varied from 5.4 to 37.8 vol % by varying the feed ratio of PHPS to the epoxide monomer. The reaction mechanism of epoxy group and PHPS was investigated by using glycidyl methacrylate as a model compound of the epoxy monomer by H-1-nucular magnetic resonance and Fourier transform infrared spectrometry. Ammonia gas provided by the decomposition of PHPS with moisture converted PHPS to silica and cured the epoxy monomer. The curing of epoxy monomer preferably proceeded than the conversion of silica. The addition of 1,4-diaminobutane drastically accelerated the rate of curing; white and hard epoxy resinsilica nanocomposites were obtained. The good thermal stability of the composite prepared with DGEBA/PHPS/1,4-diaminobutane was observed by thermogravimetric analysis. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013