Electronic absorption and 7-T magnetic circular dichroism (MCD) spectra in the UV-visible region are reported for the binuclear diplatinum(III) ions: Pt2(SO4)4X2n-, n = 2, X = H2O, NH3; n = 4, X = Cl-, Br-, OH-. Pt2(HPO4)4X2n-, n = 2, X = H2O, Me2S; n = 4, X = Cl-, Br-. Both the absorption spectra and the MCD spectra for complexes with the same axial ligand X are similar for oxo-bridging SO42- or HPO42-. However, the MCD spectra show marked differences depending upon the nature of the X ligand. The most intense absorption band observed in the UV for the diaqua complexes at 4.4-mu-m-1 exhibits a negative A term, while that for the dichloro complexes at 3.3-mu-m-1, or for the dibromo complex at 2.95-mu-m-1, shows a positive A term. The intense bands in the spectra of the Me2S complex at 2.96-mu-m-1, the NH3 complex at 3.84-mu-m-1, and the OH- complex at 3.62-mu-m-1 show positive B terms. The absorption and MCD spectra are interpreted in terms of ligand to metal change transfer (LMCT) from bridge oxygens for the diaqua complexes and in terms of pi-LMCT from the axial halide ligands for the dihalo complexes. The intense bands for the Me2S, NH3, and OH- complexes are ascribed to (X-sigma, d-sigma) --> d-sigma* excitation. For each type of complex a lower energy, less intense band system is also observed. This system is not very strongly dependent upon the nature of X and is assigned as d-pi* --> d-sigma* or sigma*-PtO4 (predominantly d(x)2-y2).