The Bacillus subtilis oxalate decarboxylase (E.C. 4.1.1.2), YvrK, converts oxalate to formate and CO₂. YvrK and the related hypothetical proteins YoaN and YxaG from B. subtilis have been successfully overexpressed in Escherichia coli. Recombinant YvrK and YoaN were found to be soluble enzymes with oxalate decarboxylase activity only when expressed in the presence of Mn salts. No enzyme activity has yet been detected for YxaG, which was expressed as a soluble protein without the requirement for Mn salts. YvrK and YoaN were found to catalyse minor side reactions: oxalate oxidation to produce H₂O₂ and oxalate-dependent, H₂O₂-independent dye oxidations. The oxalate decarboxylase activity of purified YvrK was O₂-dependent. YvrK was found to contain between 0.86 and 1.14 atoms of Mn per subunit. EPR spectroscopy showed that the metal ion was predominantly but not exclusively in the Mn(II) oxidation state. The hyperfine coupling constant (A = 9.5 mT) of the main g = 2 signal was consistent with O and N ligands with hexacoordinate geometry. The structure of YvrK was modelled on the basis of homology with oxalate oxidase, canavalin and phaseolin and its hexameric oligomerisation was predicted by analogy with proglycinin and homogentisate 1,2-dioxygenase. Although YvrK possesses two potential active sites, only one could be fully occupied by Mn. The possibility that the C-terminal domain active site has no Mn bound and is buried in an inter-subunit interface within the hexameric enzyme is discussed. A mechanism for oxalate decarboxylation is proposed, where both Mn(II) and O₂ are cofactors that act together as a two-electron sink during catalysis.