Abstract The reactions of N2H4 and N2H4 · H2O respectively, with [N(C4H9)4][Fe(S2C6H4)2] are investigated. With N2H4 · H2O the complex salt [N(C4H9)4]2[μ-N2H4{Fe(S2C6H4)2}2] is obtained, the first hydrazine complex of a dithiolato transition metal system, which can be fully characterized. The structure shows, that the compound consists of discrete cations and anions; in the centrosymmetrical anions two [FeS4] centres are bridged by a hydrazine ligand with Fe-N distances of 224 pm, N-N distances of 142 pm and Fe-N-N angles of 116.1°. The iron atoms are square pyramidally configurated and located 35 pm above the plane determined by the S atoms.