Electrochemical measurements are presented for acidic solutions containing alpha-H4SiW12O40 adsorbed on Ag(111) and Au(111) electrodes. These measurements show that this molecule passivates the Ag surface towards solution redox events. This passivation is unique to Ag, as it is not observed on Au or carbon electrodes. Depassivation can be accomplished by moving the potential of the Ag electrode into the hydrogen evolution region. These results are discussed in terms of formation of a reduced Ag-SiW12O40 complex that is stable only at negative potentials.