Twenty-one a-amino aza heterocycles were reacted with (ethoxymethy1ene) malononitrile. UV and'H and 13C NMR studies indicated that in solution the structures of the condensation products can be described as chain and ring tautomers. Investigations were also made of the solvent and temperature dependence of the position of equilibrium of the ring and chain tautomer forms and the effect of protonation on the ring-chain tautomerism of the 2- ...