Synthesis of new benzothiazolylazo indole dyes were carried out by diazotization of 2-aminobenzothiazole, 6- and 5,6-disubstituted 2-aminobenzothiazoles using with nitrosyl sulfuric acid and coupling with various 2- and 1,2-disubstituted indole derivatives. The structures of the dyes were characterized by UV-vis, FT-IR, H-1 NMR, mass spectroscopic techniques and elemental analysis. Their solvatochromic properties in different solvents were investigated and their absorption spectra were found to be solvent dependent. The azo-hydrazone tautomerism was discussed in relation to solvent and substituent effects. In addition, acid-base effect on this equilibrium was also evaluated. To determine the most tautomeric form of the prepared dyes in solid state, X-ray data for 2-Methyl-3-(6-methoxybenzothiazol-2-yldiazenyl)-1H-indole were recorded in different solvents. The X-ray results showed that the dye was stable as azo tautomer in the solid state. Published by Elsevier B.V.