Ab initio molecular orbital calculations at various, correlated levels of theory have been performed on eight isomers of the [H,O-4,S-2](-) anion and six forms of [H-2,O-4,S-2]. For the former species, the hypothetical O3SSOH- (9) ion is identified as the lowest energy isomer using second-order Moller-Plesset perturbation theory or the CCSD(T) method, with the protonated dithionite ion, O2SSO2H- (6), being only marginally (less than 5 kJ mol(-1) at 25 degrees C) higher in energy, In contrast, the B3LYP density functional predicts a rather different order of relative stabilities, with O2SSO2H- (6) as the by far most stable ion, 6 is stable with respect to dissociation into SO2 and HOSO- or into SO2.- and HOSO. (which is more stable than HSO2.). For the species of composition H2S2O4 the cagelike dithionous acid molecule (HO)O2SSO2(OH) (14) represents the global minimum at all four levels;of theory. However, also for this species, the relative stabilities predicted by the B3LYP approach differ considerably from results obtained from the conventional wave function-based methods. 14 is stabilized by two very strong intramolecular hydrogen bonds (symmetry C-2) The homolytic dissociation of 14 at the SS bond is strongly endothermic, but the molecule is unstable with respect to dissociation into H2SO2 and SO2. The harmonic wavenumbers and infrared intensities of the fundamental vibrations of 6 and 14 are given.