When Ag2O/Ag-coated quartz crystal was exposed to flowing iodide solution formation of AgI was monitored by an electrochemical quartz crystal microbalance. The corrosion potential of Ag2O/Ag in iodide solution decreased from the equilibrium potential of Ag2O/Ag to the final equilibrium potential of AgI/Ag as shown in Figure 1. Mass change due to the conversion between Ag2O and AgI was monitored. It was clear that the conversation rate mainly occurred in the early stage. A galvanic coupling between Ag2O/Ag and AgI/Ag forms in the following stage. Influence of iodide concentration on the conversion rate will be discussed in detail based on eQCM results.