Conversion of NO to other nitrogen oxides is an elementary step in its catalytic removal processes. On coinage metal surfaces, two kinds of NO activation mechanisms have been well documented: the unimolecular dissociation of NO generates two adsorbed atoms, and the dissociation of an adsorbed (NO)(2) unit generates an adsorbed O and a free N2O. In this work, we observed a disproportionation mechanism involving three NO molecules on Au-6(-) at a very low temperature (150 K), in which an adsorbed (NO)(2) reacts with a free NO forming an adsorbed NO2 and a free N2O. The density functional theory (DFT) calculations indicated that this disproportionation step is significantly exothermic and has a very low activation barrier. The charge distributions on the involved cluster complexes and the correlation between the activity and the electronic properties of Au-6(-) indicate the important role of extra negative charge in all reaction steps. The disproportionation mechanism revealed in this work could possibly exist in the NO removal processes on real gold catalysts.