A series of cationic and neutral Ru(II) complexes of the general formula [Ru(L)(X) (tBuCN)(4)](+)X(-) and [Ru(L)(X)(2)(tBuCN)(3))], that is, [Ru(CF(3)SO(3)){NCC(CH(3))(3)}(4)(IMesH(2))](+)[CF(3)SO(3)](-) (1), [Ru(CF(3)SO(3)){NCC(CH(3))(3)}(4)(IMes)](+)[CF(3)SO(3)](-) (2), [RuCl{NCC(CH(3))(3)}(4)(IMes)](+)Cl(-) (3), [RuCl{NCC(CH(3))(3)}(4)(IMesH(2))(+)Cl(-)]/[RuCl(2){NCC(CH(3))(3)}(3)(IMesH(2))] (4), and [Ru(NCO)(2){NCC(CH(3))(3)}(3)(IMesH(2))] (5) (IMes=1,3-dimesitylimidazol-2-ylidene, IMesH(2)=1,3-dimesityl-imidazolin-2-ylidene) have been synthesized and used as UV-triggered precatalysts for the ring-opening metathesis polymerization (ROMP) of different norborn-2-ene- and cis-cyclooctene-based monomers. The absorption maxima of complexes 1-5 were in the range of 245-255 nm and thus perfectly fit the emission band of the 254 nm UV source that was used for activation. Only the cationic Ru(II)-complexes based on ligands capable of forming μ(2)-complexes such as 1 and 2 were found to be truly photolatent in ROMP. In contrast, complexes 3-5 could be activated by UV light; however, they also showed a low but significant ROMP activity in the absence of UV light. As evidenced by (1)H and (13)C NMR spectroscopy, the structure of the polymers obtained with either 1 or 2 are similar to those found in the corresponding polymers prepared by the action of [Ru(CF(3)SO(3))(2)(IMesH(2))(CH-2-(2-PrO)-C(6)H(4))], which strongly suggest the formation of Ru-based Grubbs-type initiators in the course of the UV-based activation process. Precatalysts that have the IMesH(2) ligand showed significantly enhanced reactivity as compared with those based on the IMes ligand, which is in accordance with reports on the superior reactivity of IMesH(2)-based Grubbs-type catalysts compared with IMes-based systems.