Ferrous ion oxidation by Thiobacillus ferrooxidans was completely inhibited by 10 m M each of thiosulfate, sulfite, metabisulfite, bisulfite, and tetrathionate. The inhibition was enhanced in a low pH medium (pH 1.5 versus pH 2.5). Oxygen uptake measurements with Fe2+ as the electron donor confirmed the toxicity of thiosulfate, but also indicated its dependency on the concentration of Fe2+. Cytochrome spectra of intact cells of T. ferrooxidans showed that metabisulfite, and thiosulfate to a lesser extent, directly reduced electron transport components, in contrast to no direct reduction of cytochromes by tetrathionate and sulfite. © 1987 Springer-Verlag New York Inc.