The synthesis of some novel, hetarylazopyrazolone dyes was achieved by diazotisation of five heterocyclic amines using nitrosyl sulphuric acid, coupling with 3-methyl-1-(3', 5-dipiperidino-s-triazinyl)-5-pyrazolone. Visible absorption spectra of the dyes were examined in various solvents and the compounds in solution exhibited several equilibria. The hetarylazopyrazolones readily undergo acidic dissociation into the respective common anion in both DMF and DMSO. The absorption maxima of the dyes showed large bathochromic effects in comparison with analogue dyes containing carbocyclic amine residues and N-phenylpyrazolone derivatives. The colour of the dyes is discussed with respect to the nature of the heterocyclic ring and substituents therein. The affects of temperature, concentration as well as acid and base on the visible absorption maxima of the dyes are also reported. (C) 2002 Elsevier Science Ltd. All rights reserved.