Equilibrium constants for the addition of proton and bisulfite ion to 14 indaniline dyes were spectrophotometrically determined. determined. The pK value decreases with some exceptions almost linearly as the λ_max increases. The fact is explained by considering the stability of each of the cononical structures. The equilibrium constant for the addition of bisulfite ion increases as the absorption maximum shifts toward longer wave lengths. This can be interpreted from the electronic point of view if we assume that the addition takes place at position 3 of the naphoquinone ring, disagreeing with the conclusion of Reeves and Tong. We cannot explain, however, why the adduct will not isomerize to accomplish aromatization as the authors suggested that it would.