Abstract The 1H-,13C- and 15N-NMR spectra of three azo dyes prepared by coupling 4-nitrobenzenediazonium chloride with J-acid under both alkaline and acidic conditions have been measured. Two-dimensional H, H-COSY; NOESY; H, C-COSY and one-dimensional selective INEPT have been used in the assignment of the 1H- and 13C-NMR signals. Using δ(13C), δ(15N) and 1J(15N15N) it has been found that the coupling products ortho and para to the hydroxyl group exist, in hexadeuteriodimethyl sulfoxide solutions, as equilibrium mixtures of the azo and hydrazone tautomers with strongly prevailing naphthaquinone phenylhydrazone configurations, whilst the coupling product ortho to the amino group exists almost exclusively as the azo compound.