The ground states of FeS2 and FeS2-, and several low-lying excited electronic states of FeS2 that are responsible for the FeS2(-) photoelectron spectrum, are calculated. At the B3LYP level an open, quasi-linear [SFeS]conformation is found as the most stable structure, which is confirmed at the ab initio CASPT2 computational level. Both the neutral and the anionic unsaturated complexes possess high-spin electronic ground states. For the first time a complete assignment of the photoelectron spectrum of FeS- is proposed. The lowest energy band in this spectrum is ascribed to an electron detachment from the two highest-lying 3d(pi) antibonding orbitals (with respect to the iron-sulfur bonding) of iron. The next-lowest experimental band corresponds to an electron removal from nonbonding, nearly pure sulfur orbitals. The two highest bands in the spectra are assigned as electron detachments from pi and sigma bonding mainly sulfur orbitals.