We introduce a novel theoretical approach for determining oxidation states (OS) from quantum-mechanical calculations. For a transition-metal ion, for example, the metal-ligand orbital mixing contribution to the charge allocated to the ion is separated from that due to the actual occupation of the d-orbitals from which OS can then be inferred. We report the application of this approach to different transition-metal systems: molecular complexes, ruthenium-dye molecules, ruthenium complexes with noninnocent ligands, and bulk semiconductors. The computations were carried out using density-functional theory with a Hubbard U correction. The oxidation states were determined without ambiguity.