Acrylonitrile (AN) was polymerized in the presence of poly(methyl methacrylate) (PMMA) seed latex by both the batch and dropwise addition modes. The two addition modes both led to PMMA/polyacrylonitrile (PAN) composite particles with a core/shell structure with PMMA as the core and PAN as the shell. The shell thickness could be adjusted by a change in the amount of the second-stage AN monomer relative to the PMMA seed polymer. However, the surface morphologies of the composite polymers were quite different, which showed a rambutan-like batch mode and an urchinlike dropwise mode. The reason was considered as the different precipitation mechanisms of PAN small-particle precipitation for the batch mode and linear PAN segment growth for the dropwise mode. 0 2008 Wiley Periodicals, Inc. J Appl Polym Sci 112: 410-415, 2009