Catalytic performance of limonite in the decomposition of 2000 ppm NH3 in the presence of a small concentration of H2S has been studied with a cylindrical quartz reactor at a high space velocity of 45000 h(-1) to examine the sulfur tolerance of the limonite catalyst. It is not poisoned apparently by 50-500 ppm H2S at 750 degrees C, but the remarkable sulfur poisoning is observed at 2000 ppm H2S. In the coexistence of 100 ppm H2S, conversion of NH3 to N-2 decreases gradually with time after 10 h at 650 degrees C, whereas the limonite maintains the high conversion of almost 100% for 50 h at 750 degrees C. The SEM-EDX measurements after reaction show that the surface composition at 750 degrees C is richer in metallic Fe, which may be produced by the reaction of NH3 and/or H-2 with FeS formed. Commercially-available FeS promotes the decomposition of NH3 diluted with inert gas at 750 degrees C, and the FeS is partly transformed into H2S and alpha-Fe in this process. Possible mechanisms for the decomposition of NH3 in the presence of H2S with the limonite are discussed on the basis of the results of some model experiments. (C) 2007 Elsevier B.V. All rights reserved.