The structure of the title compound, 7-methoxy-2-methyl-4,5-dihydroxyanthracene-9,10-dione, C16H12O5, was originally reported by Ulickýet al. [Acta Cryst. (1991). C47, 1879–1881] in the space group P212121 [polymorph (Io)]. The new polymorph, (Im), crystallizes in the space group P21/c. The molecular structures are closely similar, with both –OH groups forming intramolecular hydrogen bonds to one of the neighbouring quinone O atoms, thus slightly lengthening this C=O bond; the pattern of C—C bond lengths in the ring system is consistent with some contribution from a resonance form with a negative charge at the hydrogen-bonded quinone O atom and an aromatic region around its neighbouring C atoms. The packing of (Im) is simpler than the extensively crosslinked pattern of (Io), with molecular tapes connected by classical (but three-centre) and `weak' hydrogen bonds, parallel to [20].