A series of hetarylazoindole dyes were obtained by coupling 2-methyl, 2-phenyl, 1,2-dimethyl and 1-methyl-2-phenylindoles with diazotized 2-amino-1,3,4-thiadiazoles and 3-amino-5-mercapto-1,2,4-triazole. The dyes were characterized by UV-vis, FT-IR, H-1 NMR, LC-MS or MS spectroscopic techniques and elemental analysis. The solvatochromism of the dyes was evaluated with respect to visible absorption properties in various solvents. The colours of the dyes were discussed with respect to the nature of heterocyclic rings and to the substituents therein; the effects of acid and alkali on the visible lambda(max) of the dyes are also reported. In addition, X-ray diffraction analysis of the crystal structure of 2-phenyl-3-(5-ethyl-1,3,4-thiadiazol-2-yldiazenyl)-1H-indole revealed that the dye was in the azo form and that intra- and intermolecular N-H center dot center dot center dot N hydrogen bonds link the molecules to form infinite chains in the  direction. The asymmetric unit of the dye contained two independent molecules. (c) 2007 Elsevier Ltd. All rights reserved.