AbstractGlassy composites were made by photopolymerization of a dimethacrylate paste containing silanated silicate particles (up to 50 μm). Minimum values of Vickers hardness number, stiffness, and yield stress were found for volume fractions of the filler in the range 0.03 to 0.05. Generally fracture surface morphology resembled classical results for inorganic glasses. In addition, tracks and features were observed of the kind reported previously for highly crosslinked organic glasses. Such morphology has been attributed to fortuitous changes in direction of the principal tensile stress just ahead of the crack tip. In the present work it was found that single tracks and features were associated with individual filler particles. This observation is consistent with the expectation that individual particles would act as stress concentrators and thereby serve to change the direction of the principal stress. Furthermore, stress concentration may be invoked to account for the minimum in mechanical properties but dewetting might also be involved