To elucidate the formation process of precursor of gold-supported manganese dioxide (MnO2), the coprecipitation behavior of [AuCl4-n(-)(OH)(n)](-) (n = 0-4) (Au(III)) complex ions with manganese(II) hydroxide (Mn(OH)(2)) and the change in their chemical state were examined. The Au(III) complex ions were rapidly and effectively coprecipitated with Mn(OH)(2) at pH 9. According to the Mossbauer spectra for gold (Au) coprecipitated with Mn(OH)(2), below an An content of 60 wt% in the coprecipitates, all of the coprecipitated An existed in the atomic state (Au(0)), while, above an An content of 65 wt%, part of the gold existed in the Au(III) state, and the proportion increased with increasing coprecipitated An content. Based on the results of X-ray photoelectron spectroscopy, Mn(II) in Mn(OH)2 converted to Mn(IV) in conjunction with coprecipitation of Au(III) complex ions. These results indicate that the rapid stoichiometric reduction of Au(III) to Au(0) is caused by electron transfer from Mn(II) in Mn(OH)(2) to the Au(III) complex ion through an Mn-O-Au bond. (c) 2007 Elsevier Inc. All rights reserved.