Three series of epoxy/SiO2 composites, containing 0.3-7 wt % nanosized SiO2 with different specific surface area, were prepared by solution blending. The resulting composites exhibit the higher glass transition temperature (T-g) than that of pure epoxy. The T-g of composite showed a maximum increment of 35.3 degrees C by the addition of 7 wt % A300. The trade name of A300 is Aerosil 300. It is one of the fumed silica nanoparticles products of Degussa. The decomposition temperatures (T-d) of composites were always higher than that of pure epoxy and showed a maximum increment of 20.8 degrees C by the addition of 5 wt % A300. The light transmittance of composites was as a function of the SiO2 content and size. The water permeability of composites decreased with increasing SiO2 content and the 7 wt % A300 composite exhibits a maximum decrement percentage of 35.6%. The T-g, T-d, storage modulus, and water-vapor barrier property are as a function of the SiO2 content and size. These properties increased as the content Of SiO2 increased. The finer SiO2 are more effective in increasing the T-g, T-d, and water-vapor barrier property. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 2648-2653, 2010