Valence tautomerism is a type of tautomerism in which single and/or double bonds are rapidly formed and ruptured, without migration of atoms or groups. It is distinct from prototropic tautomerism, and involves processes with rapid reorganisation of bonding electrons.
A pair of valence tautomers with formula CHO are benzene oxide and oxepin.
Other examples of this type of tautomerism can be found in bullvalene, and in open and closed forms of certain heterocycles, such as organic azides and tetrazoles, or mesoionic münchnone and acylamino ketene.
Valence tautomerism requires a change in molecular geometry and should not be confused with canonical resonance structures or mesomers.