Copper MFI zeolites with different Si/Al ratios and different copper loadings, prepared by ion exchange, were characterised by XRD, H-2-TPR and NO TPD. The results indicated the existence of diverse copper species in CuMFI catalysts, such as isolated Cu2+, Cu+ ions and CuO species, whose concentrations depends on the catalyst Si/Al ratio and copper loading. In underexchanged catalysts (catalysts with low Si/Al ratio or copper loading) is the copper mainly present as isolated Cu2+ ions, while in overexchanged catalysts (catalysts with high Si/Al ratio or copper loading) CuO species, Cu2+ and Cu+ ions were detected. CuMFI with high Si/Al ratio or high copper loading are more easily reduced than CuMFI with low Si/Al ratio or low copper loading. An easier reducibility of the copper species has an effect on the operating temperature window of the catalysts by decreasing the temperature of the maximum NO conversion. CuMFI zeolites with a high concentration of isolated Cu2+ ions exhibit the highest NO adsorption capacity. The results make evident that the isolated Cu2+ ions are the most active species for NO SCR by propene.