Magnetic circular dichroism (MCD) and absorption spectroscopies have been used to probe the electronic structure of [PPh4][MoO(p-SC6H4X)(4)] (X = H, Cl, OMe) and [PPh4][MoO(edt)(2)] complexes (edt = ethane-1,2-dithiolate). The results of density functional calculations (DFT) on [MoO(SMe)(4)](-) and [MoO(edt)(2)](-) model complexes were used to provide a framework for the interpretation of the spectra. Our analysis shows that the lowest energy transitions in [(MoOS4)-O-V] chromophores (S-4 = sulfur donor ligand) result from S --> Mo charge transfer transitions from S valence orbitals that he close to the ligand field manifold. The energies of these transitions are strongly dependent on the orientation of the S lone-pair orbitals with respect to the Mo atom that is determined by the geometry of the ligand backbone. Thus, the lowest energy transition in the MCD spectrum of [PPh4][MoO(pSC(6)H(4)X)(4)] (X = H) occurs at 14 800 cm(-1), while that in [PPh4][MoO(edt)(2)] occurs at 11 900 cm(-1). The identification of three bands in the absorption spectrum of [PPh4][MoO(edt)(2)] arising from LMCT from S pseudo-sigma combinations to the singly occupied Mo 4d orbital in the xy plane suggests that then is considerable covalency in the ground-state electronic structures of [MoOS4] complexes. DFT calculations on [MoO(SMe)(4)](-) reveal that the singly occupied HOMO is 53% Mo 4d(xy) and 35% S p for the equilibrium C-4 geometry. For [MoO(edt)(4)](-) the steric constraints imposed by the edt ligands result in the S pi orbitals being of similar energy to the Mo 4d manifold. Significant S pseudo-sigma and pi donation may also weaken the Mo equivalent toO bond in [MoOS4] centers, a requirement for facile active site regeneration in the catalytic cycle of the DMSO reductases. The strong dependence of the energies of the bands in the absorption and MCD spectra of [PPh4][MoO(p-SC6H4X)(4)] (X = H, Cl, OMe) and [PPh4][MoO(edt)(2)] on the ligand geometry suggests that the structural features of the active sites of the DMSO reductases may result in an electronic structure that is optimized for facile oxygen atom transfer.