Anthralin 1 and some of its C-10 or O-alkylated derivatives were investigated by UV/VIS- and fluorescence spectroscopy in different solvents and buffer systems, respectively. The effects of substituents on the formation of anthralin anion as well as the constitution of the resulting anions confirm that C-H acidity at position 10 is necessary for the formation of a fully aromatic anionic form. It is concluded that the resulting anion 1a is the pharmacologically active species of the antipsoriatic anthralin. Tautomerism of the neutral molecule is not observable.