1,4,5-Trihydroxy-9,10-anthraquinone and its substituted derivatives exist in equilibrium of structures distinguished by quinoid tautomerism and rotational isomerism. Their electron absorption spectra contain π1, π*-bands corresponding to 9,10-and 1,10-, more seldom to 1,5-and 1,4-anthraquinoid structures. Of three isomeric 1,10-anthraquinones only 4,8,9-trihydroxy-1,10-antraquinones were found. All tautomer may exist as conformers with contiguous CO and OH groups not bound by an intramolecular hydrogen bond. The considerable difference in color of structurally similar substituted compounds is due to tautomerism and conformer transformations. © Nauka/Interperiodica 2007.