CASPT2 calculations reveal that the ligand field splitting parameter Delta(o) of [Os(CN)(6)](3-) is much higher than previously proposed values of +/- 38 000 cm(-1). In line with the expected increase down a transition-metal group, Delta(o) is found to be +/- 55 000 cm(-1), excluding the possible appearance of ligand field transitions in the UV-vis spectrum. Instead, the calculations confirm that the observed spectrum arises from the three lowest symmetry-allowed ligand-to-metal charge-transfer (LMCT) excitations. Spin-orbit coupling in the ground state is found to be about 4350 cm(-1), leading to a spin-orbit coupling constant zeta of +/- 2900 cm(-1). Spin-orbit coupling in the T-2(1u) LMCT states is found not to be negligible, contrary to previous belief.