The origin of the effect of nonfaradaic electrochemical modification of catalytic activity (NEMCA) or electrochemical promotion was investigated via temperature-programmed desorption (TPD) of oxygen from polycrystalline Pt films deposited on 8 mol% Y2O3-stabilized ZrO2 (YSZ), an O2-conductor, under high-vacuum conditions and temperatures of 600 to 900 K. Oxygen was adsorbed both via the gas phase and electrochemically, as O2-, via electrical current application between the Pt catalyst film and a Au counter electrode. Gaseous oxygen adsorption gives a single adsorption state (T-p approximate to 720-730 K) but electrochemical or mixed gaseous-electrochemical adsorption was found to cause significant oxygen backspillover from the YSZ solid electrolyte onto the Pt surface and the creation of two distinct oxygen adsorption states, i.e., a strongly bonded ionic state (T-p approximate to 750-780 K) and a weakly bonded state (T-p approximate to 675-685 K). The creation of these two states is also manifest by potentiometric work function measurements and high temperature cyclic voltammetry. These results provide a straightforward explanation of the effect of electrochemical promotion on Pt deposited on O2- conducting solid electrolytes. The observed pronounced catalytic rate enhancement in electrochemical promotion studies is due to the high reactivity of the weakly bonded oxygen state, while strongly bonded ionic oxygen acts asa sacrificial promoter. The binding strength and average dipole moment 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 in agreement with recent ab initio quantum mechanical calculations.