Ab initio molecular orbital theory has been used to study the three lowest open shell states of ethylenedione, C 2 O 2 , 2-thiooxoethen-1-one, C 2 OS, and ethylenedithione, C 2 S 2 . To treat the singlet and triplet states in an even-handed manner, multiconfigurational self-consistent field theory (MCSCF), which included all the important configurations for a quantitative description of these states, was used as the basis of the investigation. Further correlation effects have been included using a multireference configuration interaction (MRCI) approach. Basis sets of triple-quality with d-and f-polarization functions were employed. Equilibrium geometries were obtained from density functional theory (DFT) calculations using the B3LYP exchange correlation functional and are presented for the 3 Σ g -, 1 ∆ g states of all three molecules. Harmonic frequencies are also presented for these states and were calculated at the MCSCF and DFT levels. The reported relative energies for the states were obtained from MRCI calculations. As expected from previous work, the ground states are confirmed to be of 3 Σ g -symmetry for all molecules, but it is only for the C 2 S 2 that this state lies below the energy of the dissociation products in their own ground states. For C 2 OS, though, this energy gap is small (5.7 kcal/mol). In contrast to a number of other calculations, the 1 ∆ g states for all three molecules are also shown to be minima on the MCSCF potential energy surfaces. The 1 ∆ g states are all close to the ground states (<10 kcal/mol). Consequently, with C 2 S 2 , this singlet state also lies below the ground state of the dissociated products. The relative energies of the 1 Σ g + states at the optimized geometries for the 1 ∆ g states have also been determined.