The composition range of polyurethane (PUR)/poly(ethyl methacrylate) (PEMA) interpenetrating polymer networks was investigated with respect to morphology and phase continuity using mechanical and dynamic mechanical methods and transmission electron microscopy (TEM). Dynamic mechanical data revealed one main tan delta transition with a shoulder for the intermediate compositions from 70:30 to 40:60 indicating a semi-miscible system. For the remaining compositions only one peak, indicating a higher degree of miscibility was observed. The storage and elastic moduli were related to the Davies, Kerner and Budiansky modulus-composition models. The Budiansky modulus-composition model, which indicates phase inversion at the mid-range composition, resulted in the best fit. However, it was found that the shape of the modulus versus composition curves was strongly temperature-dependent. In previous studies, not much attention had been given to the temperature at which the modulus-composition studies were conducted. Tensile testing revealed a strong synergistic effect at the 70:30 PUR/PEMA composition with maxima occurring at this composition for both the elongation at break and the toughness index. The tensile strength increased in a three-step regime corroborating the dynamic mechanical thermal analysis results. TEM micrographs confirmed a co-continuous system at the 70:30 to 40:60 PUR/PEMA mid-range compositions.