It is mysterious that, when A. ferrooxidans ATCC 23270 cells grow on elemental sulfur, they have high iron oxidase activity comparable to that of iron-grown cells as well as high activities of sulfide:ferric ion oxidoreductase (SFORase) and tetrathionate hydrolase. To clarify this interesting phenomenon, cytochrome c and tetrathionate hydrolase were purified from sulfur-grown A. ferrooxidans cells using ammonium sulfate precipitation, Phenyl column chromatography, and SuperdexTM 75 and Sephadex G-100 size exclusion column chromatographies. The purified cytochrome c was reduced by tetrathionate in the presence of purified tetrathionate hydrolase, but not in the absence of the enzyme. When the partially purified cytochrome c fraction containing aa3-type cytochrome oxidase was used, both cytochrome c and aa3-type cytochrome oxidase were reduced by tetrathionate in the presence of purified tetrathionate hydrolase. These results indicate that tetrathionate in the presence of tetrathionate hydrolase can reduce iron oxidase enzyme system containing cytochrome c and aa3-type cytochrome oxidase as tetrathionate hydrolase decomposes tetrathionate to produce thiosulfate, elemental sulfur, and sulfate; and the formed thiosulfate can chemically reduce cytochrome c and Fe3+.