The hydrogenation of quinoline (Q) under mild conditions was studied using commercial Pd, Rh or Ru/Al2O3 catalysts. Using Pd and Rh/Al2O3, the main product obtained was the partially hydrogenated 1,2,3,4-tetrahdroquinoline (py-THQ), while Ru/Al2O3 was inactive. The synthesis of the fully hydrogenated decahydroquinoline (DHQ) was not observed, since the irreversible adsorption on the active sites of a reaction intermediate(s) formed in the step, Q --> py-THQ. No modification was observed in this adsorption with increasin g temperature and pressure or with the addition of a Bronsted acid or base. On the contrary, the addition of a strong and sterically hindered Lewis base allowed the partial formation of DHQ, hampering the irreversible adsorption of intermediate(s). (C) 2002 Elsevier Science B.V. All rights reserved.