Epoxy binders containing microballoons ranging from 38.5 to 57.7% by volume were cast in a mold and cured, and the resulting slabs were sectioned to yield short-beam test coupons. The strength changes with microballoons content were noted. These reveal an increase from 3.87 to 5.79 MPa for a decrease in microballoons content from 57.7 to 38.5%. Mechanical data were correlated with fractographic features employing scanning electron micro-scope. The failure features involving the microballoons and interface regions are highlighted in this article. The works show the existence of a dependence of strength parameters on the inter-microballoon distances. (c) 2005 Wiley Periodicals, Inc.