TheeffectofthecoadsorptionofCOandO2ontheZiff-Gulari-BarshadsurfacecatalyticreactionsysteminstudiedbyMonteCarlosimulation.ThecoadsorptionofbothspeciesaddsanextrareactionsteptotheclassicalZiff-Gulari-Barshadmodel.Itisshownthatthesecond-orderphasetransitionfromthereactivestatetotheO-passivatedstateintheZiff-Gulari-Barshadmodeliseliminated,andtheproductionrateofCO2increaseslinearlyalongthefractionycoofCOingasphasewhenitislow,inagreementwithexperimentalresults.WealsofindthattheincreaseoftheprobabilityofthecoadsorptionleadstothedecreaseofthecriticalvalueofycoofthediscontinuousphasetransitiontotheCO-passivatedstate.