Imidazolin-3-oxides I were reduced with NaBH4 in THF at reflux to give the corresponding 2,3,5-triarylimidazolidin-l-oles 2, which are proved to be in a ring-chain-ring tautomeric equilibrium with N-2-aminoethyl nitrones 3 and 3,5,6-triphenyl-1,2,5-oxadiazinanes 4. The ratios of the ring and chain form are determined by the substituent at the reaction centre and can be described by the equation logK(x) = p sigma(+) + logK(x=H). These are the first examples of a novel three-component ring-chain-ring tautomeric equilibrium characterized by a Hammett-type equation. The stability of the ring form was favoured by electron-withdrawing substituents. Treatment of the equilibrium mixture of 2, 3 and 4 with phenylisocyanate in refluxing toluene gives selectively the corresponding O-carbamoylated imidazolidines 5; cis-5 was shown to isomerize to trans-5 on heating. (C) 2007 Elsevier Ltd. All rights reserved.