Haematoxylin and its oxidation products were studied by t.l.c., and i.r., n.m.r., u.v. and visible spectroscopy. Oxidation (‘ripening’) of Haematoxylin with oxygen, from the atmosphere or a cylinder, gave Haematein as the only major product; whereas with non-gaseous oxidants such as hydrogen peroxide, potassium permanganate and sodium iodate, further oxidation readily occurred. Such ‘over-ripening’ always produced the same major product: a yellowish quinone-carboxylic acid, which we term ‘Oxyhaematein’. Oxyhaematein did not form a complex with aluminium ions under staining conditions but did act as an acid dye. Thus the deleterious effects of the overripening of Haematoxylin and the reported yellow-orange acidophil background staining sometimes seen with metal-Haematoxylin stains may both be explained. However, an additional, or alternative, explanation is available for this orange staining: acidic solutions of Haematein were found to react with acidophils of tissue sections to give an orange-brown product, not extracted by the processing fluids.