A series of simultaneous interpenetrating polymer networks, IPNs, based on a polyurethane and an unsaturated polyester resin is studied. The curing process was followed using differential scanning calorimetry (DSC) and Fourier-transform infrared spectroscopy (FTIR). The IPNs were seen to crosslink completely and the kinetics of the curing process were modified greatly, accelerating with respect to the pure homopolymers. The process of styrene-polyester copolymerization varies from an azeotropic copolymerization in the pure polyester state to alternating copolymerization in the IPNs. Miscibility, phase continuity and phase separation are studied by dynamic mechanical thermal analysis (DMTA). In general, the IPNs obtained have a high degree of interpenetration and are semi-miscibles. Intermediate compositions are slightly less miscible than the outer ranges with a transition for the second component being apparent. The empirical loss modulus-composition curves are compared with those predicted by various theoretical models. In general, the IPNs follow the Budiansky model, which predicts a phase inversion at intermediate compositions. Compositions which are rich in a specific component show a continuous phase with the disperse minority component and the intermediate compositions show two co-continuous phases. It is also seen that these curves depend strongly on the temperature at which they are formed. The loss factor reveals strong synergism and the maximum of properties are found in a composition close to 40% in polyester. Comparison of simultaneous IPNs with sequential IPNs showed that the latter show a lesser tendency to phase separation in the systems studied.