On the basis of the electronic absorption spectra it was found that 3-methyl-1-phenyl-4-phenylazo-5-pyrazolone exists in acidic solutions in mono- and diprotonated forms. It was established by means of calculations by the Pariser-Parr-Pople (PPP) method that in the neutral and protonated forms the quinone hydrazone tautomer of the phenylazopyrazolone is more stable than the azo tautomer, and it was determined that the protonation center is the N(2) atom of the pyrazolone ring. The diprotonated form is most likely the quinone hydrazone tautomer protonated at the heteroring N(2) atom and the α-nitrogen atom of the azo group. In an alkaline medium the phenylazopyrazolone exists in the form of an anion with predominance of the azo form. The effect of substituents ion the diazo component of the phenylazopyrazolone on the long-wave bands of the electronic absorption spectra in various media was explained on the basis of calculation of the distribution of the π-electron density during electron transitions. © 1992 Plenum Publishing Corporation.