1. 1-Hydroxy- or 1-oxyl-3-imidazoline 3-oxides with acyl, hydroxyalkyl, or oxime groups, or a hydrogen atom, in the 2-position of the heterocycle undergo oxidation in the presence of nucleophiles with the intermediate formation of 4H-imidazole di-N-oxides, to give nitronylnitroxyl radicals. 2. A new series of stable nitronylnitroxyl radicals with a functional group in the 4-position have been obtained. 3. The electronic effects of the nitronylnitroxyl group have been determined from measurements of the pKa values of the OH and NH2 groups in 4-hydroxy- and 4-amino-5,5-dimethyl-4-phenyl-3-imidazoline 3-oxide 1-oxyl using EPR spectroscopy.