Abstract Sulfur monoxide, SO, is a relatively unstable molecule whose metal-coordinating properties have received little attention in bioinorganic chemistry. Reported here is a density functional theory (DFT) examination of the four possible oxidation states for a heme-SO/OS adduct previously proposed to be a part of the catalytic cycle of sulfite reductases. The FeOS and FeSO isomers are found to be degenerate in energy in most cases, suggesting that they both may be observable; the FeSO isomers would be the ones more likely to occur during the catalytic cycle of sulfite reductases — a cycle which indeed is initiated with the sulfite bound to iron via the sulfur, not via the oxygen. More importantly, higher spin states tend to be favored especially in the more oxidized models — which are the states occurring earlier in the proposed catalytic cycle. This implies weaker iron–ligand bonds — and, in fact, in several cases, essentially broken bonds. The sulfite reductase active site features an iron–sulfur cluster sharing one of its sulfur thiolates with the heme as the axial ligand. This uncommon proximity has as unavoidable effect an increase in the efficiency of delivery of electrons to the heme once SO has been generated at the active site. This would then allow the catalytic cycle to proceed to the next step — exothermic protonation of the SO.