A pretreatment-transient reaction product analysis method was applied to study the reactions and average composition of the possible surface intermediate species in selective catalytic reduction with ethylene of NOx over Co-ZSM-5. The reactions of the surface species, formed by the pretreatment of Co-ZSM-5 in a NO/C2H4/O2 mixture at 275◦C, with the NO/O2 flow produced much more N2 than that with the individual NO or O2 flow. The similarity of N2/COx/H2O product distribution generated from the above surface species-NO/O2 reactions and that from the normal NO/C2H4/O2 flow reactions implies that the surface species NCaObHc formed in the three-component pretreatment process is very likely the primary intermediate surface species generated during the real flow reactions. The in situ FT-IR (DRIFT) spectroscopy measurements of the surface species support the above conclusion.