The present work reports a XPS and XAES study of Cu-Beta zeolites with different Cu loadings, coupled with an in situ type of catalytic experiments conducted in a high-pressure gas cell installed in the preparation chamber of the spectrometer, It has been found that the most active Cu-Beta zeolites for SCR of NOx are those in which the conversion between Cu+2 and Cu+1 species is quickly produced under reaction conditions, This conversion between both oxidation states is easier in over-exchanged Cu-Beta samples. When exposed to the reaction conditions, the over-exchanged Cu-Beta zeolites show a high level of dispersed Cu+1 species, and a correlation between the ratio of Cu+1 to total Cu on the surface in the working catalyst and catalytic activity has been found.