Cationic coordinatively saturated complexes of ruthenium(II), [Ru(o-C 6 H 4 -2-py)(phen)(MeCN) 2 ] þ , bearing different counterions of PF 6 À and Cl À have been used in the radical polymerization of 2-hydroxyethyl methacrylate in protic media and acetone under homogeneous conditions. Exchange of PF 6 À by Cl À increases the solubility of the complex in water.Both complexes led to the fast polymerization under mild conditions, but control was achieved only in methanol and acetone and was better for the complex with Cl À . The polymerization accelerated in aqueous media and proceeded to a high conversion even with a monomer/catalyst ¼ 2000/1, but without control. Polymerization mediated by complex bearing Cl À was slower in protic solvents but faster in acetone and always resulted in lower molecular weight polymers. Thus, the nature of the anion strongly affected the catalytic activity of the complexes and may serve as way of fine-tuning the catalytic properties.