We report the heterodimerization of eumelanin precursors from the quantum chemical calculations using a general-purpose reactivity indicator. 5,6-Dihydroxyindole (DHI), its 2-carboxy derivative (DHICA), and related molecules are under consideration. Molecules participating in the reaction have been identified from the electron transfer probability. The heterodimerization has been demonstrated to be the electron-transfer-controlled reaction. The theoretical prediction is in good agreement with the result of the previous cooxidation of DHI and DHICA. (C) 2010 Elsevier B.V. All rights reserved.