We report the instantaneous decomposition of the small molecular gold cluster, Au(25)SG(18) (SG-glutathione thiolate) in the presence of externally added chloroaurate ions resulting in the instantaneous formation of the insoluble gold-glutathione polymer. The rate of decomposition, however, was much slower for the metal ions such as Ag+, Fe3+, Cu2+, Ni2+, Cd2+, Zn2+ and Sr2+. This chemical reactivity is shown to have no direct correlation with the electrochemical potential of the added ions. (c) 2007 Elsevier B.V. All rights reserved.