Manometric experiments of tetrathionate oxidation were conducted with crude extracts of T. thiooxidans. Tetrathionate was first enzymatically dismutated to trithionate and pen-tathionate, and the tri- and pentathionate formed were further oxidized to elemental sulfur and sulfate, in which one mole of oxygen was required for the conversion of tetrathionate of two moles. α, α'-Dipyridyl, nitroso-R salt and orthophenanthroline at a concentration of 7.5×10-3M showed completely inhibitory effects for the enzymic dismutation. The dismutation was accelerated by either ferrous, nickel or cobalt ion, but was strongly inhibited by cupric ion.