Using Cu+ luminescence enabled the identification of two main Cu sites present in the MFI matrix and reflected in the 480 and 540 nm emission bands. It has been found that the activity of Cu-ZSM-5 in NO decomposition is controlled by the concentration of the sites with the Cu+ emission at 540 nm and by the reducibility of these Cu sites controlled by the framework Si/Al ratio. These Cu sites are incorporated into the zeolite as (Cu2+OH)(+) ions, bearing after calcination an extra framework oxygen, and exchanged in the vicinity of one framework aluminum.