Syntactic foams of polybenzoxazine, containing moderately high volume percentage of glass microballoons, were prepared. The specific gravity decreased with increase in microballoon content. The disproportionate decrease in specific gravity was ascribed to entrapment of air voids during compaction. The high content of microballoon increased the possibility for air voids that tended to get accumulated. The effect of microballoon concentration on tensile, compressive, and flexural strengths of the foams was studied. Tensile and compressive properties were optimized at about 68% by volume of microballoon while flexural strength decreased marginally on increasing the microballoon content. Althought the specific tensile and compressive strength showed a maximum followed by a decrease, the specific flexural strength systematically increased with microballoon content. The increased packing density of syntactic foam of a given constituent composition increased the compressive strength. The property variation was corroborated by morphological features, as evidenced in scanning electron micrographs. The syntactic foams showed "multiple resin-neck formation" and "disc-shaped microballoon regions." The crushing of microballoons during molding was inevitable when compaction was effected to achieve a density beyond the theoretical one. Low-density syntactic foams tend to fail at lower loads because of fracturing of microballoons. (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 108:1021-1028,2008.