Methods were developed to characterize the fracture behavior of a condensation cure polysilsesquioxane network, and to toughen the network with homogeneously incorporated polydimethylsiloxane (PDMS) segments. Hydroxyl terminated PDMS short chains were end-capped with tetraethoxy silane and the end-capped segments were coupled with the oligomeric silsesquioxane bearing silanol ends. The endcapping and the coupling reactions were investigated by FT-IR, Si-29 and H-1 NMR, and GPC. The complete end-capping of the PDMS chains was achieved with minimum self-condensation and cyclization, and a complete coupling of the functionalized PDMS with the oligomeric silsesquioxane was achieved with no self-condensation of the PDMS chains. To toughen the network such a coupling reaction was necessary, otherwise PDMS chains formed a separate phase which was ineffective. Short PDMS chains and silsesquioxane oligomers were incompatible and a ternary phase diagram with toluene as the third component was constructed to define a concentration window for the coupling reaction. When homogeneously reacted into the resin network, all the PDMS chains of degrees of polymerization (DP) between 8 and 55 increased the fracture toughness, and within this range the longer chains were more effective. Ten parts of PDMS of DP 55 increased the K-1c from 0.253 to 0.456 MPa m(1/2), and G(Ic), from 34.1 to 151.11 J/m(2). TCA showed the thermal stability of the network was retained after PDMS toughening. Enhanced inelastic deformation was responsible for the increased fracture toughness. Upon re-initiation of a crack, the toughened network developed a plastic zone the size of which was consistent with the calculated zone from Irwin's model, while no evidence of such yielding was seen for the untoughened network.