Warfarin in solution apparently consists of 3 interconverting tautomeric structures, 2 of which are cyclic diastereomeric hemiketals, while the 3rd and minor component is the open-chain intermediate form. Configurations of all the tautomers and major conformations of the cyclic tautomers are assigned. The assignments are supported by comparison with the chemical shift and coupling constant parameters of structurally fixed model compounds.