The systems prepared in situ by addition of two equivalents of 1,1,1-tris(diphenylphosphinomethyl)ethane (triphos) to M2Cl2(COE)(4) (M = Rh, Ir; COE = cyclooctene) showed to be efficient and regioselective precatalysts for the hydrogenation of quinoline. For both systems, kinetic studies lead to the rate laws r = {K(1)k(2)/(1 + K-1[ H-2])}[M][H-2](2); it was proposed that the catalytically active species are the cationic unsaturated complexes [M(Q)(triphos)](+). The general mechanism involves a rapid and partial hydrogenation of these species to generate complexes of the type [M(H)(2)(Q)(triphos)](+) (isolated and characterized for M = Ir), which transfer the hydride ligands to the coordinated Q to yield species containing a 1,2-dihydroquinoline (DHQ) ligand, followed by a second oxidative addition of H-2, considered as the rate-determining step of the cycle; hydrogen transfer toward the DHQ ligand yield THQ, regenerates the active species and restarts the catalytic cycle.