The oxidation of ground-state (singlet) and triplet [Ru(bpy)(3)](2+) were studied by full quantum-mechanical (QM) and mixed quantum/classical (QM/MM) molecular dynamics simulations. Both approaches provide reliable results for the redox potentials of the two spin states. The two redox reactions closely obey Marcus theory for electron transfer. The free energy difference between the two [Ru(bpy)(3)](2+) states amounts to 1.78 eV from both QM and QM/MM simulations. The two methods also provide similar results for the reorganization free energy associated with the transition from singlet to triplet [Ru(bpy)(3)](2+) (0.06 eV for QM and 0.07 eV for QM/MM). On the basis of single-point calculations, we estimate the entropic contribution to the free energy difference between singlet and triplet [Ru(bpy)(3)](2+) to be 0.27 eV, which is significantly greater than previously assumed (0.03 eV) and in contradiction with the assumption that the transition between these two states can be accurately described using purely energetic considerations. Employing a thermodynamic cycle involving singlet [Ru(bpy)(3)](2+), triplet [Ru(bpy)(3)](2+), and [Ru(bpy)(3)](3+), we calculated the triplet oxidation potential to be -0.62 V vs the standard hydrogen electrode, which is significantly different from a previous experimental estimate based on energetic considerations only (-0.86 V).