The selective oxidation of hydrogen sulfide to sulfur was studied over iron-molybdenum oxides with various Fe-Mo ratios. Strong synergistic phenomenon in catalytic activity was observed for the Fe-Mo-O binary oxides. Under identical reaction conditions, the areal rates of the binary oxides were superior to those of the corresponding single oxide catalysts, which suggest that the new compound Fe-2(MO4)(3) formed in the binary oxide is more active than Fe2O3 and MoO3. The oxidation rates of H2S were found to exhibit first-order dependence on the hydrogen sulfide concentration, which implies that the activation of H2S is the rate-limiting step.