The dependence of the deactivation rate of Cu/ZSM-5 lean NOx reduction catalysts on the composition of the feed was studied. No catalyst deactivation was observed in the absence of H2O at 400 degrees C after 160 h. Catalyst deactivation was much faster if the feed contained C3H8, NO, O-2, and H2O (lean NOx reduction conditions) than if one of these components was missing. Fresh and deactivated catalysts were characterized by a variety of techniques, including EPR, XRD, BET surface area measurements, IR spectroscopy, and temperature programmed reduction with H-2 (H-2-TPR). With some techniques, the differences between fresh and deactivated catalysts were marginal, but very significant changes were observed with TPR and EPR. No gross destruction of the zeolite framework was detected by XRD or BET, even for a catalyst that was 50% deactivated. Isolated CU2+ ions, [Cu-O-Cu](2+) oxocations and CuO particles were identified in fresh Cu/ZSM-5. In deactivated Cu/ZSM-5, the Cu species were redistributed. Besides isolated CU2+ ions (in two different coordination environments), highly dispersed Cu ions in Al2O3 and a CuAl2O4 compound were detected. Ion exchange of Cu2+ into partially dealuminated H/ZSM-5 yielded TPR and EPR evidence strikingly similar to deactivated Cu/ZSM-5. These results suggested that the formation of proton exchange sites plays a crucial role in catalyst deactivation. In the presence of steam, it induces dealumination of the zeolite, and the copper states are irreversibly changed due to copper interacting with the alumina formed in the dealumination process.