Simultaneous interpenetrating polymer networks (SINs) of poly(allyl diglycol carbonate) (ADC) and a polyurethane (PU) were synthesized. The effects of the network composition and the crosslink density of the PU phase on the morphology, mechanical properties and thermal transition behaviour of the PU/ADC SIN were studied. At 10% composition, dynamic mechanical analysis revealed complete phase miscibility. As the PU content increased, although phase separation occurred, the samples retained their optical transparency. Modulus and tensile strength decreased, whilst elongation increased, as the PU content increased. At 30% PU content, transmission electron micrograph studies revealed a co-continuous phase morphology, and the domain sizes increased as the PU crosslink density increased. Modulus, tensile strength and fracture toughness also increased with increasing crosslink density.