Catalytic reduction of SO2 to elemental sulfur by CO has been systematically investigated over gamma-Al2O3-supported sulfide catalysts of transition metals including Co, Mo, Fe, CoMo and FeMo with different loadings of the metals. The sulfided CoMo/Al2O3 exhibited outstanding activity: a complete conversion of SO2 was achieved at a temperature of 300 degrees C. The reaction proceeds catalytically and consistently over time and most efficiently at a molar feed ration CO/SO2 = 2. A precursor CoMo/Al2O3 oxide which experienced sulfurization through the CO-SO2 reaction yielded a working sulfide catalyst having sulfides decreased in order of 4% Co 16% Mo > 4% Fe 15% Mo > 16% Mo greater than or equal to 25% Mo > 14% Co greater than or equal to 4% Co > 4% Fe. A DRIFT study showed that CO adsorbs exclusively on CoMo phase and that SO2 predominantly on gamma-Al2O3. It is suggested that the Co-Mo-S structure is more adequate than the other metal-sulfur structures for the formation of a carbonyl sulfide (COS) intermediate because of the proper strength of metal-sulfur bond, and catalytically works with gamma-Al2O3 for the COS-SO2 reaction. (C) 2000 Elsevier Science B.V. All rights reserved.