In situ electron paramagnetic resonance (EPR)s pectroscopy was applied to dilute copper chabazite (CHA) zeolites under gas flows relevant for the selective catalytic reduction of NO with ammonia (NH 3 -SCR). Under both reducing and oxidizing conditions, we observed differences in reactivity between the differentm onomeric coppers ites present:U pon reduction of Cu 2 + with NO + NH 3 ,t he rate is seen to dependo nt he NH 3 coverage. Subsequent oxidation with O 2 resulted in ac lean EPR spectrum of only one type of copper site, whereas oxidation with NO + O 2 gave two types of copper sites. The rate of oxidation differed significantly between the reaction with O 2 alone and with NO + O 2 together.T hus, it was revealed that [Cu(NH 3 ) 2 ] + complexes,w hich are regarded to be only weakly associated with the framework, nevertheless have different reactivity depending on the Al distribution in the proximity. The observedd ifferences in reactivity of the coppers ites have implications for the mechanistic understanding of NH 3 -SCR with Cu zeolites.The characterization of copper-exchangedz eolitesh as been performed with al arge variety of spectroscopic methods. Efforts have been driven by the fact that copper-exchanged zeolites are excellent catalysts in the reduction of NO x (NO and NO 2 )t oN 2 by selective catalytic reduction with ammonia (NH 3 -SCR), which today is one of the fastest growing applications for zeolite materials. Specifically,s mall-pore zeolites, such as chabazite (CHA), show excellent activity,h ydrothermal stability, and hydrocarbon tolerance and low production of unwanted N 2 O. [1][2][3] Al arge selection of experimental methods (IR spectroscopy, X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES), X-ray diffraction (XRD), and UV/Vis spectroscopy) [4][5][6][7] and theoretical approaches (DFT) [8][9][10] have been appliedi nr ecent years to elucidate the properties of Cu-CHA materials. EPR spectroscopy offersu nique opportunities be-cause of its high spectral resolution, its fundamentally different selection rules, and its applicability under in situconditions. [11,12] Recent studies on identicalC u-CHAm aterials revealed several well-defined Cu sites. [5-7, 13, 14] This complicates detailed mapping and kinetic analysiso ft he SCR reaction on chabazite zeolites, for which the discussion has mostly focusedo nt he difference between 2Al Cu sites and 1Al Cu sites. It is of vital importance to detailed kinetic studies [15] to be able to distinguish and characterize all sites. Herein, we present an experimental protocola nd as pectroscopicm ethod that are able to reveal significant differences between different types of 2Al Cu sites present.T he reactivity is followed under SCR-relevant in situ conditions, and the results give important insight into the reactivity at single Cu sites.After dehydration under an O 2 -rich atmosphere at temperatures above 200 8C, Cu 2 + loses coordinated water molecules and binds directly to the zeolite framework. The three most important monomeric si...