The graft copolymerization of acrylic acid onto methylcellulose by ceric ion/p-xylene redox pair was investigated in aqueous media under homogeneous conditions. The graft yield dependency on p-xylene concentration in the range 1.8-45.0 X 10(-5) M showed a minimum and an enhanced yield when the methylcellulose interacted with ceric ion and p-xylene for an initial period of 10 min (preoxidation time) prior to addition of monomer to the reaction medium. This was attributed to the presence of two kinetically controlled reactions initiated by p-xylyl radical and diradical species. At prolonged preoxidation times of 30 and 60 min, the graft yield dependency on p-xylene concentration was normal and suggested the presence of only one initiating species. The effect of ceric ion on the graft reaction in the concentration range of 8.33-83.3 X 10(-3) M was optimal at 131% graft yield for ceric ion concentration of 16.7 X 10(-3) M and was reduced significantly by as much as 75% at the highest concentration of the latter. The temperature dependency of graft yield was negative in the region 30-50degreesC. At 50degreesC the initial rate of graft was only 37% of the value at 30degreesC.