Numerous microorganisms oxidize sulfur for energy conservation and contribute to the global biogeochemical sulfur cycle. We have determined the 1.7 angstrom–resolution structure of the sulfur oxygenase reductase from the thermoacidophilic archaeon Acidianus ambivalens , which catalyzes an oxygen-dependent disproportionation of elemental sulfur. Twenty-four monomers form a large hollow sphere enclosing a positively charged nanocompartment. Apolar channels provide access for linear sulfur species. A cysteine persulfide and a low-potential mononuclear non-heme iron site ligated by a 2-His-1-carboxylate facial triad in a pocket of each subunit constitute the active sites, accessible from the inside of the sphere. The iron is likely the site of both sulfur oxidation and sulfur reduction.