Synthesis of some novel hetarylazocoumarin dyes was achieved by diazotisation of nine heterocyclic amines using nitrosyl sulphuric acid, coupling with 4-hydroxy-2H-1-benzopyran-2-one (4-hydroxycoumarin). These dyes were characterized by UV-vis, FT-IR and H-1 NMR spectroscopic techniques and elemental analysis. Both the effect of varying pH and solvent upon the absorption ability of azocoumarins are discussed using visible absorption spectra of the dyes. The color of the dyes is discussed with respect to the nature of the heterocyclic ring and to the substituent therein. The effects of temperature, concentration as well as acid and base on the visible absorption maxima of the dyes are also reported. (C) 2004 Elsevier Ltd. All rights reserved.