The microstructure of an adhesive can have a significant effect on the fracture of bonded joints. In the present work, the effect of large second-phase filler particles (approximately 3 to 100-mu-m) has been studied using a scanning electron microscope fitted with an in situ straining stage. Observations of the fracture of bonded joints and bend bars containing blunt notches have been combined with stress analysis to determine the fracture stress of the filler particles.