Novel simultaneous interpenetrating polymer networks (SINs) of poly(allyl diglycol carbonate) (ADC) and a rigid high modulus polyurethane based on the Conathane UC-33 prepolymer system (UC) were synthesized. The completion of each component reaction was verified by means of Fourier transform IR spectroscopy. The effects of the composition on the morphology, mechanical properties, and thermal transition behavior of the SINs were studied. At 10% composition, transmission electron microscopy revealed complete phase miscibility. This was also supported by the high optical transparency of the sample. As the UC content increased to 20% a two-phase morphology was observed in which the UC phase domains were dispersed in the matrix of the ADC phase. At higher UC content the ADC continuous phase was replaced by the UC phase, implying the occurrence of phase inversion. This variation of morphology allowed the SINs to have a property range from the UC-toughened plastics to the ADC-reinforced polyurethane. The SINs containing 50% UC showed synergism in the mechanical properties as the elongation was significantly increased to 4 times that of the unmodified UC and ADC homopolymers.