The origin of the effect of non-faradaic electrochemical modification of catalytic activity (NEMCA) or Electrochemical Promotion was investigated via temperature-programmed-desorption (TPD) of oxygen, from polycrystalline Pd films deposited on 8 mol% Y2O3-stabilized-ZrO2 (YSZ), an O2- conductor, under high-vacuum conditions and temperatures between 50 and 250 degrees C. Oxygen was adsorbed both via the gas phase and electrochemically, as O2-, via electrical current application between the Pd catalyst film and a Au counter electrode. Gaseous oxygen adsorption gives two adsorbed atomic oxygen species desorbing at about 300 degrees C (state beta(1)) and 340-500 degrees C (state beta(2)). The creation of the low temperature peak is favored at high exposure times (exposure >1 kL) and low adsorption temperatures (T-ads < 200 degrees C). The decrease of the open circuit potential (or catalyst work function) during the adsorption at high exposure times, indicates the formation of subsurface oxygen species which desorbs at higher temperatures (above 450 degrees C). The desorption peak of this subsurface oxygen is not clear due to the wide peaks of the TPD spectra. The TPD spectra after electrochemical O2- pumping to the Pd catalyst film show two peaks (at 350 and 430 degrees C) corresponding to spillover O-ads and O-ads(delta-) according to the reaction: O-(YSZ)(2-) -> O-ads(delta-) -> O-ads The formation of the spillover O-ads(delta-) oxygen species is an intermediate stage before the formation of the atomic adsorbed oxygen, O-ads. Mixed gaseous and electrochemical adsorption was carried out in order to simulate the Electrochemical Promotion conditions. The initial surface coverage with oxygen from the gas phase plays a very important role on the high or low effect of polarization. In general mixed adsorption leads to much higher oxygen coverages compare with that observed either under gaseous or electrochemical adsorption. The binding strength of the atomic adsorbed oxygen (state beta(2)) was investigated as a function of applied potential. It was found that the binding energy decreases linearly with increasing catalyst potential and work function. Similar behavior has been observed for oxygen adsorption on Pt, Ag and Au deposited on YSZ in previous studies.