A high temperature thermosetting aryl dicyanate resin was modified with thermoplastic polysulfone and/or poly(ether imide) at various compositions. Differential scanning calorimetry was used to characterize the phase behaviour, while the morphologies and continuities of the thermoplastic/dicyanate networks were investigated by using dynamic mechanical analysis, scanning electron microscopy, fracture toughness testing, and solvent sorption studies. The phase structure was found to change from a thermosetting continuity to a co-continuity of both thermoplastic and thermosetting domains. The mechanical and physical properties of the modified polycyanates exhibited a step change at the shift of the phase structure.