Syntactic foams based on a reactive polymer blend of Epoxy-Allyl Phenol-Bismaleimide were processed using hollow glass microballoons. The effect of microballoon concentration on the mechanical and thermophysical properties of the foam composites was investigated. The mechanical properties, specific strength, coefficient of linear expansion, and density of the foam composites showed decreasing trend with increase in microballoon concentration whereas the specific heat showed a marginal increase. The temperature dependence of the thermophysical properties revealed an increase in coefficient of linear expansion, whereas the specific heat was more or less insensitive to the test temperature. Syntactic foams of fixed filler loading (50% by weight) were processed using a blend of two types of microballoons of different shell thickness. The mechanical properties increased proportional to the concentration of the higher shell thickness microballoon. At moderate filler load microballoon breakage dominated the failure mode. The compositional dependency of the mechanical properties was corroborated by SEM analysis of the fracture surfaces. (c) 2007 Wiley Periodicals, Inc.