A high-molecular weight condensed tannin-like purple pigment, hordeumin, was formed from the fermented broth of barley bran. The formation of hordeumin was promoted in light and an oxidation state from the fermented filtrate containing hordeumin precursors. When hydrogen peroxide that promoted the formation of a hydroxyl radical was added to the fermented filtrate, the formation of hordeumin was accelerated with an increase in deoxyribose oxidation. The formation of hordeumin was suppressed by the Fe-ion chelating agent DETAPAC and several hydroxyl radical scavengers. Based on these results, it is assumed that the formation of hordeumin is an oxygen-mediated reaction and that reactive oxygens participate in one of these reactions.