An effective toughening approach is described in this report. This approach uses a combination of both short and long PDMS segments, simultaneously incorporated into a polysilsesquioxane-based rigid network through chemical bonding at the terminals of these segments. Upon curing, the short chain PDMS remains molecularly dispersed (phase I PDMS) and the long chain PDMS segregates to form silicone rubber particles in situ (phase II PDMS). Proper combinations of phase I with phase II PDMS toughen the network 7-9 times more effectively than the phase I alone at the same total PDMS loading level, while the phase II PDMS alone deteriorates the mechanical properties. The effectiveness of the phase I/II combinations is dependent on the particle size. Submicron-sized particles are more effective than particles of a few microns in diameter. Particles larger than a few tens of microns become ineffective. The size of the particles can be controlled by changing these parameters: the precoupling reaction conditions, the amount of phase II PDMS, the phase II PDMS chain length, and the chain length ratio of phase I PDMS segment to phase II PDMS segments. With appropriate phase I/II combinations, the K-Ic is increased by up to 220% and the G(Ic) by up to 900%, with less loss of elastic modulus as compared with toughening by the phase I alone. It is proposed that the effective engagement of high cross-link density domains into the deformation process by submicron-sized rubbery particles is responsible for the increased fracture toughness.