AbstractThe thermal expansion behavior of a cyanate ester matrix reinforced by fumed silica nanoparticles with average primary particle diameters of 12 and 40 nm was investigated with thermomechanical analysis. All nanocomposites showed decreased coefficients of thermal expansion (CTEs) in comparison with the neat bisphenol E cyanate ester resin, but the 12‐nm fumed silica nanocomposites had lower CTEs than the 40‐nm nanocomposites for equal volume fractions. The largest decrease in CTE was 27.0% for 20.7 vol % 40‐nm fumed silica. When the data were compared to applicable theory, the best fit of the data was given by Schapery's upper limit and Shi's model. Estimates of the interphase volume fraction and effective thickness surrounding the nanoparticles were made with the results of Shi's model, and the results showed that the interphase volume fraction was larger for the 12‐nm fumed silica nanocomposites, given an equal fraction of silica. The glass‐transition temperature of the nanocomposites from thermomechanical analysis varied only slightly with the volume fraction. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008