The relative energies of a multitude of low-lying electronic states of Fe2S2-/0/+/2+ have been determined by complete active space self-consistent-field (CASSCF) calculations. For selected states dynamical correlation has been included by the multireference configuration interaction method (MRCI) and the structures of some high-spin states have been optimized by CASSCF/MRCI. Comparison is made with structures obtained by density functional calculations. In all oxidation states of Fe2S2 the numerous states are assigned to spin ladders. The ground states of Fe2S2-/0/+/2+ are (10)A(g), (1)A(g), B-2(1u) and (1)A(g), respectively. The total splittings of the lowest-energy spin ladders of Fe2S2, Fe2S2-, and Fe2S2+ are about 0.17, 0.18, and 0.35 eV, respectively. The spin ladders of Fe2S2 qualitatively reflect the picture of Heisenberg spin coupling. Some of the spin ladders of Fe2S2+ show the picture of combined Heisenberg coupling and double exchange. The calculated adiabatic electron affinity of Fe2S2 is 1.1 eV (observed 2.1 eV). Our estimate of the ionization energy of Fe2S2 is 7.9 +/-0.5 eV. An interpretation of the observed photoelectron spectrum of Fe2S2- is given.