In this, the second of two papers on a series of simultaneous polyurethane (PUR)/polystyrene (PS) interpenetrating polymer networks (TPNs), the T-g behaviour, mechanical properties and modulus-composition relations have been studied. A gross phase morphology over the full IPN composition range was indicated by two separate loss factor peaks from dynamic mechanical thermal analysis (DMTA). Both DMTA and modulated-temperature differential scanning calorimetry (MT-d.s.c.) measurements revealed that the T-g of the PS transition increased with decreasing PS content in the IPN. This was explained by an increase in interactions between the PUR hard segments and the pi-electrons of the PS phenyl rings. Despite the phase-separated morpholo_gy, materials with good mechanical properties were obtained. The tensile properties and the Shore hardness results were comparable to similar semi-miscible PUR/PEMA IPNs. The Budiansky modulus-composition relation resulted in the best fit with the experimental data, indicating phase inversion at mid-range compositions. Modulus-composition studies, indicating that phase inversion occurred at the 30:70 PUR/PS IPN composition, corroborated the electron microscopy findings from the first paper in this series.