Usingfirst-principlescalculations,wesystematicallystudythedissociationsofO2moleculesondifferentultrathinPb（111）films.AccordingtoourpreviousworkrevealingthemolecularadsorptionprecursorstatesforO2,wefurtherexplorewhytherearetwonearlydegenerateadsorptionstatesonPb（111）ultrathinfilms,butnoprecursoradsorptionstatesexistingatallonMg（0001）andAl（111）surfaces.Thereasonisconcludedtobethedifferentsurfaceelectronicstructures.FortheO2dissociation,weconsiderboththereactionchannelsfromgas-likeandmolecularlyadsorbedO2molecules.WefindthattheenergybarrierforO2dissociationfromthemolecularadsorptionprecursorstatesisalwayssmallerthanthatfromO2gas.ThemostenergeticallyfavorabledissociationprocessisfoundtobethesameondifferentPb（111）films,andtheenergybarriersarefoundtobeinfluencedbythequantumsizeeffectsofPb（111）films.
