Spectral properties, solvatochromism and azo-hydrazone tautomerism of ten 5-(substituted phenylazo)3-cyano-6-hydroxy-1-(2-hydroxyethyl)-4-methyl-2-pyridones in twenty-two solvents are investigated. For quantitative evaluation of the solvent effects on the UV-vis absorption maxima, the principles of the linear solvation energy relationships are used, i.e. models proposed by Kamlet-Taft and Catalan. Linear free energy relationships are applied to the UV-vis absorption spectra and correlation of absorption frequencies with Hammett substituent constants are performed. Furthermore, the influence of the electronic nature of the substituents on H-1 and C-13 NMR shifts is investigated by simple and extended Hammett equations, as well as by Swain-Lupton equation. (C) 2014 Elsevier B.V. All rights reserved.