A series of doubly-charged [4Fe4S] cluster ions, [Fe 4 S 4 X 4 ] 2− (X = Cl, Br, I, and SC 2 H 5), were produced in the gas phase by electrospray ionization and examined using collision-induced dissociation (CID) in a quadrupole ion trap. CID channels of each dianion were analyzed by MS/MS experiments at different collision energies. Two CID channels were observed for these dianions: symmetric fission and electron detachment. The most interesting observation is the symmetric fission of [Fe 4 S 4 X 4 ] 2− into two identical daughter ions, [Fe 2 S 2 X 2 ] − . The important role of the intramolecular coulomb repulsion in the symmetric fission was shown by comparing with CID of the singly-charged anions, [Fe 4 S 4 X 4 ] − . Our gas-phase experiments suggest that solution phase conversions between [Fe 4 S 4 ] and [Fe 2 S 2 ] assemblies in proteins may also involve related fission chemistry with reactive [Fe 2 S 2 X 2 ] − intermediates.