The kinetic scheme of free radical graft copolymerization of acrylonitrile (AN) onto sodium alginate, using ceric ammonium nitrate as an initiator, has been proposed and equations relating the values of Rp, Rr and Rh are also suggested. The results have been found to be in good agreement with the proposed mechanism. The per cent grafting of AN onto sodium alginate has also been determined as a function of composition of the mixed solvents comprising methanol: water, ethanol: water, n-propanol:water and n-butanol: water systems. The reaction medium has been found to play an important role in graft copolymerization. The results of the effect of the reaction medium on grafting are also discussed.