The static fracture toughness behavior of glass filled epoxies was studied using single-edge-notched 3 point bend (SEN-3PB) tests. Three different types of glass reinforcements, large glass spheres (LGS), small glass spheres (SGS), and glass fibers (GF), were examined. The surface of each type of filler was treated to either promote or inhibit particle-matrix adhesion. The volume fraction of filler studied ranged from 10 to 30 vol%. Special attention was given to the effect of particle surface treatment and/or moisture exposure on fracture toughness. Interestingly, moisture exposure resulted in improved toughness in the case of poor adhesion at the matrix-particle interface for LGS and GF filled epoxies. Transmission optical microscopy and scanning electron microscopy were used to examine the process zone in SEN-3PB specimens. These observations along with yield stress measurements suggest that the improved toughness was due to enhanced shear yielding of the matrix due to poor adhesion at the matrix-particle interface and increased matrix ductility due to the plasticizing effect of the absorbed moisture. (C) 2003 Elsevier Science Ltd. All rights reserved.