A series of experiments were designed and conducted to determine the significance of process parameters in the grafting of styrene and acrylonitrile onto polybutadiene seeds in a semicontinuous emulsion copolymerization system. The significances of the parameters were obtained by comparing the variance ratios, or F values, with F-distributions. The significance level of each test (alpha-value) was obtained by variance analysis. The important process parameters in industrial polymerization processes are usually monomer-to-polymer ratio, initiator type and concentration, chain-transfer agent, and reaction temperature. The target responses were final monomer conversion, grafting degree, grafting efficiency, gel percent, and viscosity-average molecular weight of free styrene-acrylonitrile (SAN). The analysis of variance indicated that cumene hydroperoxide as the initiator and reaction temperature had strong effects on the graft structure. Moreover, free SAN molecular weight was significantly affected by the monomer/polymer ratio and cumene hydroperoxide and n-dodecyl mercaptan as chain-transfer agents. The raspberry-like morphology of grafted acrylonitrile-butadiene-styrene (ABS) particles and phase separation within the particles were confirmed by transmission electron microscopy. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 1752-1761, 2011