Void toughening is studied using epoxy resin. Voids were produced mechanically without any chemical agents so that it was possible to isolate the role of micro-voids from other factors. Toughness of the epoxy resin owing to voids was improved over that of the control by an order of magnitude. The toughening mechanism was found to be strongly related to intervoid distance and, hence, to void-to-crack distance. Large deformation bands between the crack and voids were deduced to be the source of toughening. The intervoid distance was directly measured on scanning electron microscope photos. (c) 2005 Wiley Periodicals, Inc.