The visible electronic spectral behaviour of Eriochrome Black T, in a number of organic solvents of different characteristics has been examined. It was concluded that the dye exists in solution in basic solvents (based on their autoprotolysis constant, pKs) in an azo hydrazone tautomeric equilibrium, while in acetone, acetonitrile and DMSO it exists mainly in the azo form. In DMF and other basic solvent solutions, the dye exhibits an acid-base equilibrium of the type H2In HIn2− + H+. The proportional concentration of the base form (HIn2−), as well as the relative tautomerisation, are largely dependent upon the concentration of Erio T, the basicity of the solvent and the tendency of the base and/or the hydrazone form to be stabilized by H-bonding interaction with the solvent molecules. The compound was found to aggregate in water and hydroxylic solvents. The bands displayed by the differents forms of the compound have been assigned to an electronic transition involving the whole molecule with an appreciable charge transfer character.