The electronic absorption spectra of a series of 4-aryl-azopyridylpyrazolones were investingated in different organic solvents of different polarities. The ir-spectra analysis suggested the presence of enol-azo and hydrazono-keto tautomeric structures. The1H-nmr-spectra of these compounds were analysed and assigned in term of these proposed tautomeric structures. An additional proof of this finding was given by mass spectral fragmentation analyses.