A versatile photoactivated catalytic system based on a cyclometalated ruthenium(II) complex, composed of strongly coordinating bidentate and relatively labile ligands, in conjunction with a traditional alkyl bromide initiator, has been developed for living/controlled radical polymerization. Polymerizations of three typical hydrophobic monomersmethyl methacrylate (MMA), styrene (St), and n-butyl acrylate (BA)proceeded to high conversions under visible light irradiation. The polymerization process was photoresponsive, i.e., took place only under irradiation and immediately stopped when the light was turned off. Block copolymers of MMA with St and BA with St, as well as statistical copolymer of BA and St, could also be conveniently prepared. 1 H NMR and electrochemical studies suggest a mechanism of the catalytic activation, which involves a photoinduced formation of the solvento 18-electron species cis-[Ru(o-C 6 H 4 -2-py)(phen)(MeCN)(acetone)] + through the intermediacy of the 16-electron five-coordinated complex cis-[Ru(o-C 6 H 4 -2-py)(phen)(MeCN)] + which is believed to be a crucial intermediate of the overall ATRP process.