The oxidation of the two catechol rings A and B in the chemical structure of hematoxylin in aqueous solution has been studied theoretically in order to identify the mechanism of oxidation. In a recent experimental study, the oxidation mechanism of hematoxylin was designated an E rC iE r process in which an irreversible chemical reaction (C i) followed the reversible chemical electrochemical oxidation (E r) of the catechol unit connected to the six-membered ring of the molecule (ring A). The theoretical results presented herein indicate that the electrochemical oxidation of ring B is actually slightly more favoured than ring A, although the potential separation is so small that they were unable to be distinguished in the experimental study. We therefore suggest that the most likely mechanism is E rE rC iE r, in which two reversible electrochemical oxidation reactions (E r) occur preceding the irreversible chemical reaction (C i), though we cannot rule out a contribution from E rC iE r. The calculated oxidation potential (0.719V v. standard hydrogen electrode) is in close accord with the experimental value (0.759V v. standard hydrogen electrode). The deprotonation of five hydroxyl groups of hematoxylin in aqueous solution is also studied and the order of acidic strength of these groups has been identified.