In the work of Zhu and co-authors a series of azo naphthols, derived from aminohydroxy naphthalenesulfonic acids (so called J-and Gamma-acids), are described. All studied compounds follow the same pattern a-c of substituents in the naphthyl ring and various substituents in position 5 in the phenyl (OCH 3 , CH 3 , H, SO 3 Na and CF 3 with numbering 1-5, respectively) ring. In the current note 3a-c (see Table ) will be used as representative examples. Since there is no previous experimental information exactly for their tautomerism, in addition, intermediate structures 3d-g will be used to link them to the core structure 3 for which tautomeric properties substantial experimental information is available. the used experimental method, but in all cases it is noted that the compound always exists as a tautomeric mixture in solution. According to our data, obtained from UV-Vis spectroscopy the amount of the hydrazo form dominates, while the A-tautomer is in the range of 40% in ioctane to 28% in chloroform and methylene chloride (i.e. the equilibrium is unsensitive on the solvent as stated previously). In the current study the value in cyclohexane will be used to validate the theoretical approach since the compound is not water soluble. In this solvent the hydrazo form is presented in 62%, which gives a value of the Gibbs free energy at room temperature of -0.24 kcal/mol (the minus indicates the shift of the equilibrium towards H form). The crystal structure of 3, in which the hydrazone form is estimated roughly to 75%, is also known. According to the theoretical data, summarized in Table , the hydrazo tautomer is more stable in cyclohexane by -0.33 kcal/mol as Gibbs free energy, which is in an excellent agreement with the experiment. This result is a confirmation of the of the well-known ability of M06-2X to predict tautomeric state in azonaphthols and related Schiff bases and gives a reason the same density functional to be used here. As seen from Table , the same value in water should be -0.83 kcal/mol, which corresponds to the general stabilization of the more polar H by increasing solvent polarity. Since the dyes 3a-c are newly synthesized compounds and there is no information about their tautomerism, the simplified models 3d-h are used to link them to the core structure 3.