The effect of the presence of different amounts of block copolymers [polystyrene-block-poly(metliyl methacrylate)] on the morphology of polystyrene/poly (methyl methacrylate) composite latex particles was investigated. The block copolymers were produced in situ by controlled radical polymerization (CRP) through the addition of the second monomer to a seed prepared by miniemulsion polymerization with a certain amount of a CRP agent. With an increase in the amounts of the block copolymers, the particle morphology changed from a hemisphere morphology (for a latex without block copolymers, i.e., without the use of a CRP agent during the polymerization) to clear core-shell morphologies as a result of decreasing polymer-polymer interfacial tension. (C) 2006 Wiley Periodicals, Inc.