The origin of the effect of nonfaradaic electrochemical modification of catalytic activity (NEMCA) or electrochemical promotion was investigated via temperature-programmed desorption of oxygen from polycrystalline Ag films deposited on 8 mol% Y2O3-stabilized ZrO2 (YSZ), an O2- conductor, under high-vacuum conditions and temperatures of 200 to 400 degrees C. Oxygen was adsorbed both via the gas phase and electrochemically, as O2-, via electrical current application between the Ag catalyst film and a Au counterelectrode. Gaseous oxygen adsorption gives a single adsorption state (T-p approximate to 290-335 degrees C) at low adsorption temperatures with subsurface oxygen (T-p approximate to 500 degrees C) also forming for higher adsorption temperatures (T > 350 degrees C). Electrochemical or mixed gaseous-electrochemical adsorption was found to cause significant oxygen backspillover from the YSZ solid electrolyte onto the Ag surface and the creation of two distinct oxygen adsorption states, i.e., a strongly bonded ionic state and a weakly bonded state. The strongly bonded state (T-p approximate to 380-400 degrees C) is identical to the one formed via electrochemical adsorption. The weakly bonded state (T-p approximate to 320-340 degrees C) desorbs at temperatures similar with the gaseous adsorption state. The creation of these two states is also manifest by high temperature cyclic voltammetry. Mixed adsorption leads to much higher oxygen coverages than gaseous adsorption. The binding strength of the weakly bonded oxygen state was investigated as a function of applied potential. It was found that the binding energy of adsorbed oxygen decreases linearly with increasing catalyst potential and work function. The results suggest that, similarly to the case of Pt, the non-Faradaic rate enhancement observed in NEMCA studies of catalytic oxidations on Ag/YSZ is due to the creation of two adsorption states, of which the weakly bonded one is highly reactive while the strongly bonded one acts as a sacrificial promoter. The fact that, unlike Pt, the Tp of the weakly bonded state is not lower than that of oxygen formed via gaseous adsorption can account for the less pronounced NEMCA behavior of Ag vs Pt catalysts.