The selective oxidation of hydrogen sulfide containing excess water and ammonia was studied over vanadium-antimony mixed oxide catalysts. The investigation was focused on the phase cooperation between V-Sb-O and Bi2O3 in this reaction. Strong synergistic phenomenon in catalytic activity was observed for the mechanically mixed catalysts of V-Sb-O and Bi2O3. Temperature-programmed reduction (TPR) and oxidation (TPO), two separated bed reaction tests, and XPS analyses were carried out to explain this synergistic effect by the reoxidation ability of Bi2O3. (C) 2003 Elsevier B.V. All rights reserved.