The graft copolymerization of styrene and methyl methacrylate onto natural rubber latex was studied under various reaction conditions using a cumene hydroperoxide redox initiator. The monomer conversion, graft copolymer compositions, and grafting efficiency were determined. The synthesized graft copolymers were purified and then characterized by proton nuclear magnetic resonance (H-1-NMR) analysis and differential scanning calorimetry (DSC). A 2(IV)(6-2) fractional factorial experimental design was applied to study the main effects on the grafting. The variables investigated in this work were the amount of the initiator and emulsifier, the presence or absence of a chain-transfer agent, the styrene-to-methyl methacrylate ratio, the monomer-to-rubber ratio, and the reaction temperature. The measured response for the experimental design was the grafting efficiency. The analysis of the results from the design showed the sequence of the main effects on the observed response of the grafting of styrene and methyl methacrylate onto natural rubber, in ascending order. The amount of the chain-transfer agent and the reaction temperature in the range of the test had significant effects and one marginally significant effect was the monomer-to-rubber ratio. (C) 2003 Wiley Periodicals, Inc.