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 Bi 2 O 3 in this reaction. Strong synergistic phenomenon in catalytic activity was observed for the mechanically mixed catalysts of V-Sb-O and Bi 2 O 3 . 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 Bi 2 O 3 .