Coupled oxygen transfer and electron transfer reactions that use cofactors are enzymatic reactions of crucial significance to all lifeforms from bacteria to vertebrates. In the European Journal of Inorganic Chemistry, scientists have introduced a model for the enzyme sulfite oxidase. It is based on a molybdenum complex whose special ligands can be oxidized to allow coupling of oxygen transfer to a substrate with an intramolecular electron transfer. Intermediates containing molybdenum(V)-ion play a critical role.