Bigraft terpolymers P(B-graft-MMA,S) are prepared easily through consecutive radical grafting of methylmethacrylate (MMA) and styrene (S) from polybutadiene (PB). Electron microscopy shows that both the raw products and the isolated pure graft terpolymers, which can be obtained by extraction methods, are microphase separated. In blends with the two incompatible polymers polystyrene and poly(vinyl chloride), the graft terpolymer behaves as an ideal polymeric surfactant and is located preferentially at the interfaces. The reasons for this unusual behaviour are discussed.