The influence of toughener concentration on the fracture behavior of two-phase, rubber-toughened epoxy is studied. To vary the concentration without altering other morphological features, samples generated from dispersions of preformed rubber (acrylic) particles in liquid epoxy monomer are used. By diluting with different amounts of epoxy prior to cure, the toughener concentration can be varied over a wide range. Thermal and microscope studies support the assertion of a constant morphology. The fracture results show that the toughness increases to a maximum and then decreases as the concentration is increased. This suggests an optimum concentration of toughening. Micrographs of the initiation zone on the fracture surface at high concentrations of rubber show less deformation than the equivalent surfaces at lower concentrations. This is consistent with a toughening mechanism based on particles initiating yielding and plastic flow in the matrix.