In the long-term electrolysis Of CO2 with a metal electrode, it is very important to maintain the catalytic activity of the electrode. The Faradaic efficiency for the reduction Of CO2 On an Ag foil electrode is known to decrease rapidly several tens of minutes after the start of electrolysis, owing to the deposition of graphitic carbon, adsorbed organic intermediate or oxide/hydroxide acting as a catalytic poison. In order to settle this problem, the electrochemical reduction of CO2 on a net Ag electrode was here performed at the three-phase (gas \ solid \ liquid) interface where CO2 was supplied directly from the gas phase and a high concentration of CO2 was sustainable in the course of electrolysis. The conversion Of CO2 (initial volume 219 CM) with this electrode (11.8 cm(2)) was found to reach 60% at an electrolysis time of 5 h with the selective formation of CO. Furthermore, the addition of AgNO3 to the electrolyte resulted in a 100% conversion Of CO2. This was brought about by in situ formation of the silver crystal face providing a favorable reaction site for the CO2 reduction, and the Faradaic efficiency of H-2 was kept below 8% during the electrolysis.