The hexarhenium anionic cluster complex with terminal formate ligands [Re6S8(HCOO)(6)](4-) was obtained by the room-temperature reaction between [Re6S8(HCOO)(6)](4-) and formic acid in an aqueous solution. The cluster was crystallized as a potassium or cesium salt and characterized by X-ray single-crystal diffraction and elemental analyses, IR, H-1 NMR, UV/vis, and luminescence spectroscopies. In particular, the emission quantum yield of the potassium salt of the Re-6 cluster anion in the solid phase was determined for the first time. The electronic structures of [Re6S8(HCOO)(6)](4-) and [Re6S8(HCOO)(6)](4-) were also elucidated by DFT calculations.