A flow cell was designed for in-situ ESR monitoring of the state of copper ions in a working Cu-ZSM-5 catalyst at high temperatures. The equilibrium oxidation state of the copper in the catalyst depends both on the temperature and the composition of the gas mixture. At 500 degrees C in [C3H6 (or C2H5OH) + NO + O-2] with excess oxidant, the condition prevailing during selective catalytic reduction (SCR) of NOx, virtually all the copper ions preserve the Cu2+ state. At 200-350 degrees C in the same gas mixtures a partial reduction of the Cu2+ ions is observed. Steam-aging (650-700 degrees C) or dry calcination (850-900 degrees C) of CuH-ZSM-5 lead to the change in Cu2+ coordination without noticeable agglomeration or encapsulation of the isolated cations. The drastic loss of catalytic activity due to such treatment is associated with the change in the localized coordination of the isolated cupric sites.