Abstract This paper examines the influence of adding polycarbonate/polystyrene graft copolymers to blends of a chlorinated polycarbonate and polystyrene; the backbone of the graft copolymers is the same chlorinated polycarbonate. The interaction parameter between the polycarbonate and polystyrene is calculated. The copolymer acts as an interfacial agent and its influence on the morphologies, dynamic-mechanical properties and interfacial area of the blends is investigated. Morphologies of both polycarbonate-rich and polystyrene-rich blends were determined; in one case dispersed particles of the minor phase are stabilized by polycarbonate chains with one end attached to the interface, in the other case stabilization is by polystyrene chains with two ends attached to the interface (loops). This system is one in which the component polymers are very immiscible. The results are compared with data determined previously on a system based on the same polycarbonate and poly(methyl methacrylate), which is characterized by very limited miscibility.