Interpenetrating polymer networks (IPN''s) of butadiene rubber/polystyrene were synthesized by sequential polymerization. The glass transition behaviour, dynamic modulus, density, hardness, and thermal stability of IPN''s were investigated The degree of intermixing of the component polymers was increased with lowering the average molecular weight per crosslink, Mc, and with increasing the rate of network formation of and component polymer (PS). The positive contribution of solvent for increasing the degree of intermixing was observed.