Phase separation behavior of the polyurethane (PU)-polystyrene (PS) interpenetrating polymer networks (IPN''s) was investigated by measuring the turbidity during simultaneous polymerization. The change in transmitted light intensity through the reacting mixture was recorded with elapsed time, and was studied as a function of composition, synthesis temperature, crosslink density and catalyst concentration. The polymerization kinetics of the PU and PS were also obtained for the 50/50 composition mixture. The PU polymerization rate was much higher than PS, thus it seemed that the phase separation process was mainly controlled by the PU polymerization rate. The onset of the phase separation occurred at around similar molecular weight range of the PU, regardless of other experimental variables. It was also observed that the rate of phase separation was dependent on the rate of the increase in molecular weight as well as the diffusion rate controlled by the medium viscosity.