Abstract: Reactions of ammonium molybdate tetrahydrate, ammonium thiocyanate, and hydroxylammonium chloride as a reducing agent in aqueous solution probably afforded a specie containing the Mo-N≡O fragment, which was further treated with ammonium polysulfide to result in isolation of two tetranuclear molybdenum(IV) sulfur nitrosyl complexes (NH₄)₄Mo₄(μ₄-S)(μ₃-S)²(μ-S₂)₄(η²-S₂)(NO)₄·4H₂O (1) and (NH₄)₄Mo₄(μ₄-O)(μ-S₂)₆ (NO)₄ (2). Two tetranuclear molybdenum(IV) complexes were characterized spectroscopically by ultraviolet spectrophotometer UV-3000 and infrared spectrophotometer FTS-40, and their structures were determined by single-crystal X ray diffraction, the results show that the two complexes containe nitrosyl group, their electrochemical properties were also investigated.