The first oxo-selenide tungsten cluster [W-3(mu(3)-Se)(mu-O)(3)(H2O)(9)](4+) has been prepared by means of in situ reductive selenidation of WO42- with H2Se at 60 degrees C in high yield. The aqua complex has been converted by ligand exchange into (MeImH)(5)[W-3(mu(3)-Se)(mu-O)(3)(NCS)(9)] (MeIm is N-methylimidazol), whose structure was determined by X-ray analysis, and into [W-3(mu(3)-Se)(mu-O)(3)(acac)(3)py)(3)]PF6. The compounds have been characterised by H-1 NMR, ESI-MS, and cyclic voltammetry.