AbstractChemically or photochemically generated1O2(1Δg O2) transforms the anion of dithranol (1) into chrysazine(2), probablyviaanendo‐peroxide. Under basic conditions,3O2converts1to the bisanthrone3and to dithranol brown. The anion of dithranol is a photosensitizer.1O2was trapped by 2,3‐dimethyl‐2‐butene and by methyl oleate which was converted to the C‐9‐ and C‐10‐hydroperoxides. From this point of view interference of the antipsoriatic dithranol with the metabolism of arachidonic acid ap