Abstract The oxidation of 3-hydroxyanthranilic acid (30HA) 1 by aqueous buffered potassium ferricyanide produces a number of coloured compounds. These include cinnabarinic acid 2 (yellow), a p-quinone dimer 3 (red) and 9-carboxy-2-hydroxy-3H-phenoxazin-3-one 4 (brown). Also present by tlc is a bright pink compound. The structure of this compound has been demonstrated to be triphenodioxazine-1, 8-dicarboxylic acid 5a.