Castor oil and 4,4’-diphenyl methane diisocyanate based polyurethane (PU)-polystyrene (PS) simultaneous interpenetrating polymer networks (IPNs) were synthesized with various PU/PS ratios (100/0, 90/10, 80/20, 75/25, 60/40 and 50/50). Mechanical properties (tensile strength, % elongation, elastic modulus, wear resistance, tear resistance and Shore-A hardness) of the IPNs showed a significant improvement beyond a critical styrene level (25% by weight). However, % elongation at break showed a maximum at 40% styrene. PU/PS IPNs of a fixed styrene level with varying amounts of styrene crosslinking agent, divinyl benzene (DVB), were also synthesized. These materials showed a significant improvement in the mechanical properties with increasing concentration of DVB. The observed behaviour is explained on the basis of results from swelling and Soxhlet extraction studies.