The electronic structures of organic reducing agents used in the leaching of polymetallic nodules and related organic compounds were investigated by means of INDO and CNDO methods of quantum chemistry. It was found that among the electronic structure indices studied, the highest occupied molecular orbital (HOMO) energy of an organic compound correlated best with its reducing power. The HOMO energy of oxalic acid was recommended as the criteria for the selection of effective reducing agents for the acid leaching of polymetallic nodules. (C) 2002 Elsevier Science B.V. All rights reserved.