A series of hetarylazopyrazolone dyes were prepared by coupling 3-methyl-1-phenyl-2-pyrazoline-5-one with 11 heterocyclic amines in nitrosyl sulphuric acid. Absorption spectra of the dyes were examined in various solvents and the compounds in solution exhibited hydrazone-common anion equilibrium. The hetarylazopyrazolones readily undergo acid dissociation into the common anion in methanol, DMF and DMSO. Absorption maxima of the dyes showed large bathochromic effects in comparison with analogous dyes containing carbocyclic amine residues in DMF except those containing isoxazolyl and pyridinyl moieties. The pKa values of the compounds have been determined spectrophotometrically in various ethanol-water solutions. The effect of temperature and concentration on the visible absorption spectra are also discussed. (C) 1999 Elsevier Science Ltd. All rights reserved.