Grafting of maleic anhydride (MA) onto natural rubber (NR) has been investigated by using a radical initiated grafting process. NR was masticated to lower molecular weight than the original NR before treatment with 0.5-1.5%, by mole of benzoyl peroxide (BPO) and 5-25% by mole of MA in toluene solution. Fourier transform infrared spectroscopy (FTIR) and titration method were used to analyze the fixation of MA on the NR. The reaction temperature was varied from 60 to 80degreesC. The % grafting of MA was higher at higher temperatures. Compared to the reaction without styrene, the addition of 0.1% by mole of styrene in the grafting process increased % MA grafting up to two times. It is postulated that the styrene may act as a charge transfer complex for the reaction of NR with MA. (C) 2004 Wiley Periodicals, Inc.