A proposal to define the oxidation state of an atom in a compound as the hypothetical charge of the corresponding atomic ion that is obtained by heterolytically cleaving its bonds such that the atom with the higher electronegativity in a bond is allocated all electrons in the bond. Bonds between like atoms are cleaved homolytically. This definition includes all atoms in compounds that were covered by the corresponding IUPAC definition, but is also applicable to organic molecules and inorganic compounds for which the IUPAC rules cannot easily be applied. (Contains 4 figures.)