The oxidation of thiols to corresponding disulfides by Indian Ocean ferromanganese nodules has been studied under varying experimental conditions. More than 90% conversion of thiols (2.5 x 10(-3) mol) was achieved at 35 degrees C using 0.1 g nodules, The oxides of Mn, Fe, Ca, Mg, and Al and surface oxygen in the nodules are most likely responsible for the oxidation of thiols. Under identical conditions the oxidative conversion of thiols decreases in the order 1-dodecanethiol < 1-hexanethiol < 1,4-butanedithiol < alpha-toluenethiol.