13C NMR has been used in a study of the isomers formed in azo coupling of 1,8-dihydroxy-3,6-naphthalenedisulfonic acid (chromotropic acid), 1-hydroxy-8-amino-3,6-naphthalenedisulfonic acid (H acid), 1-hydroxy-6-amino-3-naphthalenesulfonic acid (J acid), and 1-hydroxy-7-amino-3-naphthalenesulfonic acid (gamma acid); and indicators have been established through which individual isomers in mixtures can be recognized spectroscopically.