In contrast to the behaviour of supported Cu and An catalysts, the selectivity of but-2-en-l-ol in the gas-phase hydrogenation of but-2-enal at 120degreesC is not increased by pre-treatment of Co/Al2O3, CO/SiO2, Co/ZnO, Ni/Al2O3, Ru/Al2O3 and Rh/Al2O3 with thiophene. Selectivities to by-products formed by dimerisation and hydrogenolysis are, however, decreased. (C) 2002 Elsevier Science B.V. All rights reserved.