The graft copolymerization of methacrylic acid (MAA), methyl methacrylate (MMA) onto styrene-butadiene−styrene star triblock copolymer (SBS) was studied by ,60,Co γ−ray irradiation. The graft yields increased with MAA or MMA concentration, and with 1 wt% CCl,4, added to the grafting system. The structure of the graft copolymers was determined by Fourier−transform infrared (FT−IR) spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and ,13,C{,1,H} nuclear magnetic resonance (NMR). The results showed that hydroxyl radicals from, the irradiation product of water solvent, played an important role in improving the graft yield. It was found that decomposition temperature and the glass transition temperature of PS phase of the grafted polymer were improved, too, to a large extent. The study is of help to higher temperature applications of star SBS as a binder.