A novel C-S bond formation reaction took place, when a lithium phenolate derivative was treated with a disulfide-bridged dicopper(1) complex or a bis(mu-thiolato)dicopper(II) complex under very mild conditions. The reaction has been suggested to proceed via a disulfide-bridged (mu-phenoxo)dicopper(l) complex as the common reaction intermediate. Copper(II) complexes of the modified ligands containing a thioether group (products of the C-S bond formation reaction) have been isolated and structurally characterized by X-ray analysis as model compounds of the active site of galactose oxidase. Mechanism of the C-S bond formation reaction is also discussed in relation to the biosynthetic mechanism of the organic cofactor Tyr-Cys of galactose oxidase.