The influence of H+ and Na+ co-cations on the formation of Cu species and on NO reduction activity of Cu/ZSM-5 catalyst is investigated. 3.5Cu/0.6NaZSM-5 catalyst was significantly more active for NOx reduction with NH3 compared with 2.6Cu/HZSM-5. Catalyst characterization indicated that, the type of co-cations present in the catalyst tailors copper species redox properties, NO2 formation rate and acidity of the catalyst. Copper species in NaZSM-5, is relatively easily reducible than when present in HZSM-5. UV-vis and FTIR characterization showed the presence of a higher amount of Cu+ species in 3.5Cu/0.6NaZSM-5 compared to 2.6Cu/HZSM-5. The easily reducible copper along with Cu+ species in 3.5Cu/0.6NaZSM-5 seems to activate oxygen which reacts with NO to form NO2. The Na+ co-cations can further capture the thus produced NO2 to form intermediate surface nitrite/nitrate species, which improves the NOx conversion. The acidity measured by NH3-TPD and FTIR is needed for SCR, but not crucial for NOx conversion. (C) 2010 Elsevier B.V. All rights reserved.