Upon UV irradiation, the complexes [(eta(6)-arene(1))Ru(eta(6)-arene(2))](2+) (toluene-4-sulfonate or trifluormethane-sulfonate as counter ion; examples for arene : benzene, toluene, mesitylene, hexamethylbenzene, anisole, biphenyl, naphthalene) and [Ru(NC-R)(6)](2+) (tosylate or triflate as counter ion; R = methyl, ethyl, phenyl) are transformed into active catalysts for ring opening metathesis polymerization (ROMP) of strained bicyclic olefins. The photoinduced ring opening metathesis polymerization (PROMP) is most efficient with sandwich complexes having high quantum yields for the photochemically induced solvation of [(eta(6)-arene(1))Ru(eta(6)-arene(2))](2+) to [Ru(solvent)(6)](2+). With most of the complexes no (or only low) catalytic activity is observed in the absence of light. After the photolysis step, the mechanism of the polymerization is identical to ROMP reactions with thermally activated ruthenium catalysts. Good yields and high molecular weights are obtained with a catalyst concentration of 0.1-1%. A mechanistic model for the initiation is presented.