Grafting of methacrylic acid (and other water-soluble monomers such as acrylic acid and acrylamide) onto the surface of styrene-butadiene-styrene block copolymer films has been performed using polymeric hydroperoxides as initiator. These peroxides are obtained by reaction with the polymer of singlet oxygen, the production of which is photo-sensitized by anthracene. The rate of grafting is first order with respect to the monomer concentration in solution but increase of rate is observed for methacrylic acid concentrations higher than 1 mol l-1. The rate of grafting is also proportional to the initial concentration of anthracene in the film.