Temperature-programmed desorption (TPD), cyclic voltammetry (CV), and potentiometric work function (WF) measurements were used to investigate the state of adsorbed oxygen on polycrystalline Pt catalyst-electrode films deposited on ZrO2 (8 mol % Y2O3), an ionic O2- conductor, as a function of catalyst potential under ultrahigh-vacuum conditions and temperatures of 600-900 K. Oxygen was supplied both via the gas phase and electrochemically, as O2-, via current application between the catalyst film and a Au counterelectrode. Gaseous oxygen supply leads to a single adsorption state (T-p similar to 720-730 K) while electrochemical O2- supply, or mixed gaseous-electrochemical supply, causes significant oxygen backspillover from the solid electrolyte onto the Pt catalyst surface and the creation of two distinct oxygen adsorption states, i.e., a strongly bonded ionic state (T-p similar to 750-780 K) and a weakly bonded state (T-p similar to 675-685). The results provide a direct explanation for the effect of non-Faradaic electrochemical modification of catalytic activity or electrochemical promotion in catalysis when using O2- conductors.