NO TPD, H-2-TPR and XRD have been used to characterise copper-exchanged mordenites with different Si/Al ratios, copper contents and cocations. The results showed that copper is mainly in the form of isolated Cu2+ ions in CuMOR catalysts with copper exchange less than or equal to 20%, whereas at higher copper exchange CuO species are also present. These results were obtained with H and Na as cocation and were achieved by changing either the catalyst Si/Al ratio or the copper content. The data also indicate that the cocation mainly affects copper location and that copper is more easily reduced in sodium form catalysts than in protonic form.It was found that the isolated Cu2+ ions are the most effective species for NO adsorption and the most active species for NO SCR.