The oxidn. of reduced inorg. sulfur compds. to sulfuric acid is of great importance for biohydrometallurgical technologies as well as the formation of acidic (below pH 3) and often heavy metal-contaminated environments. The use of elemental sulfur as an electron donor is the predominant energy-yielding process in acidic natural sulfur-rich biotopes but also at mining sites contg. sulfidic ores. Contrary to its significant role in the global sulfur cycle and its biotechnol. importance, the microbial fundamentals of acidophilic sulfur oxidn. are only incompletely understood. Besides giving an overview of sulfur-oxidizing acidophiles, this review describes the so far known enzymic reactions related to elemental sulfur oxidn. in acidophilic bacteria and archaea. Although generally similar reactions are employed in both prokaryotic groups, the stoichiometry of the key enzymes is different. Bacteria oxidize elemental sulfur by a sulfur dioxygenase to sulfite whereas in archaea, a sulfur oxygenase reductase is used forming equal amts. of sulfide and sulfite. In both cases, the activation mechanism of elemental sulfur is not known but highly reactive linear sulfur forms are assumed to be the actual substrate. Inhibition as well as promotion of these biochem. steps is highly relevant in bioleaching operations. An efficient oxidn. can prevent the formation of passivating sulfur layers. In other cases, a specific inhibition of sulfur biooxidn. may be beneficial for reducing cooling and neutralization costs. In conclusion, the demand for a better knowledge of the biochem. of sulfur-oxidizing acidophiles is underlined.