3-Hydroxyanthranilic acid in aqueous solution can be autoxidized to yield two different products depending on the pH at which the oxidation is performed. At acidic pH the formation of cinnabarinic acid is favored while at alkaline pHs the major product is a newly characterized p-quinone dimer. Both of these oxidation products are formed at pH 7. A mechanism to account for these pH-dependent oxidations is proposed. © 1988, American Chemical Society. All rights reserved.