Tautomerism is the existence of two interconvertible isomers of a compound, usually in labile equilibrium, though in some cases isolable. It may be demonstrated by spectroscopy or by the exhibition of properties characteristic of both tautomers.

Most tautomerism is by hydrogen transfer, as with carbonyl compounds, in which the keto form (–CH–C=O) is in equilibrium with the enol form (–C=C–OH). Sugars display tautomerism between cyclic and straight-chain forms.