The system prepared in situ by addition of two equivalents of 1,2-bis(diphenylphosphino)ethane (dppe) to Rh2Cl2(COE)(4) (COE = cyclooctene) showed to be an efficient and regioselective precatalyst for the hydrogenation of quinoline (Q). This reaction showed to be independent of the Q concentration and of fractional order on H-2 and catalyst concentrations (1.5 and 0.6, respectively). The fractional order on catalyst concentration indicates that several catalytic species with different activities are present in the reaction medium; however, the cationic species [Rh (dppe)(2)](+) was the only phosphorous-containing compound detected by P-31{H-1} NMR. For the acac salt of this cationic bis(dppe) complex, a kinetic study led to the rate law r = {K(1)k(2)/(1 + K-1[H-2])}[M][H-2](2); [M(Q)(kappa(2)-dppe)(kappa(1)-dppe)](+) was proposed as the catalytically active species (CAS) of the cycle. The general mechanism involves a reversible oxidative addition of H-2 to generate a dihydrido complex, which transfers 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 yields THQ, regenerates the CAS and restarts the catalytic cycle.