The reaction of bis(dithiolene)sulfidotungsten(IV) complex with elemental sulfur yielded bis(dithiolene)(tetrasulfido)tungsten(IV) complex. A labeling experiment using S-34 has indicated that the reaction is reversible and a sulfido(trisulfido)tungsten(VI) species is involved as an intermediate. The reaction represents a model reaction of polysulfide reductase of molybdoenzymes.