The nature of the toughening of glassy polymers by rigid particles was investigated by using aligned assemblages of spherical glass particles. The particles were aligned by an electric field in a photopolymerizable monomer, which was polymerized while the field was still being applied. These materials were fractured with the aligned particle strings in three orientations with respect to the crack plane and propagation direction. The fracture toughness and fracture energy of these and of random arrangements of particles (formed without the electric field) were all higher than of the matrix alone. The increases were compared with predictions from processes on or in the immediate vicinity of the fracture plane and from those away from the fracture plane. The increases were inconsistent with those predicted by on-fracture plane processes as represented by crack pinning and bowing. But the increases did correlate with the size of the process zones, which could extend more than 100 mum away from the fracture plane. Detailed calculations showed that the increase in fracture energy arose almost completely from off-fracture plane processes of particle-matrix debonding and the accompanying local inelastic deformation of the matrix around the particles. (C) 2003 Elsevier Science Ltd. All rights reserved.