Metab. of thiosulfate and tetrathionate by Thiomonas intermedia K12, a facultatively heterotrophic, reduced sulfur compds.-oxidizing bacterium, was investigated. Expts. with the inhibitors N-ethylmaleimide (NEM), myxothiazol, and cyanide, and, addnl., the protonophore carbonylcyanide m-chlorophenolhydrazone (CCCP) were performed in short-time expts. to elucidate the degrdn. pathways. The results indicate a linked, cyclic degrdn. of these compds. via the highly reactive key intermediate disulfane-monosulfonic acid. Furthermore, anaerobic degrdn. expts., starting with tetrathionate, revealed a stoichiometry, which indicates a disproportionation of tetrathionate to thiosulfate, trithionate, and sulfate. The disproportionation allowed for cell growth.