The crystal structure of arsenite oxidase has been determined, and the enzyme found to represent a new subclass of the dimethyl sulfoxide (DMSO) reductase family of molybdenum-containing enzymes. The enzyme also possesses a [3Fe–4S] iron–sulfur center in the same subunit as the molybdenum center, as well as a Rieske-like [2Fe–2S] center in a separate, smaller, subunit. In addition to a detailed consideration of the enzyme's structure, its electrochemical and catalytic properties are discussed. Phylogenetic evidence is also discussed, suggesting that arsenite oxidase is an ancient enzyme arising before the diversification of the archaeal and eubacterial domains of life.