The selective oxidation of hydrogen sulfide in the presence of excess water and ammonia was studied over niobium-iron mixed oxide catalysts. They showed very high conversion of H2S with lower than 2% Of SO2 selectivity. The addition of niobium oxide to iron oxide increased H2S conversion until Nb/Fe atomic ratio of 1/2, then the conversion decreased as the Nb/Fe ratio increased from 1/2 to 5/1. Temperature programmed techniques and XPS studies revealed that the highest activity of Nb-Fe(1/2) was due to its highest reoxidation capability. Calcination temperature also affected the performance of Nb-Fe catalyst either by changing the reoxidation property or by forming a FeNbO4 phase. (C) 2004 Elsevier B.V. All rights reserved.