In the combined lean NO_x trap (LNT) and selective catalytic reduction (SCR) concept, the SCR catalyst can be exposed to rich conditions during deSO_x of the LNT. Aging of Cu/SSZ-13 SCR catalysts, deposited on a cordierite monolith, was therefore studied in rich, lean and cycling lean/rich operations at 800 °C (lean condition: 500 ppm NO, 8% O₂, 10% H₂O and 10% CO₂; rich condition: 500 ppm NO, 1% H₂, 10% H₂O and 10% CO₂). The structure of the catalyst was investigated by X-ray diffraction (XRD), surface area measurements and scanning transmission electron microscopy (STEM). In general, aging decreased the SCR activity and NH₃ oxidation. However, rich conditions showed a very rapid and intense deactivation, while lean aging led to only a small low-temperature activity decrease. The XRD results showed no sign of structure collapse, but the number of active sites, as titrated by NH₃ temperature-programed desorption (NH₃-TPD) and in situ DRIFTS, revealed an important loss of acid sites. NH₃ storage was significantly more depleted after rich aging than after lean aging. The Lewis sites, corresponding to exchange Cu²⁺, were preserved to some extent in lean conditions. Lean aging also decreased the enthalpy of NH₃ adsorption from −158 kJ mol⁻¹ to −136 kJ mol⁻¹. Moreover, a comparison of aging in lean-rich cycling conditions with aging only in rich conditions revealed that adding lean events did not hinder or reverse the deactivation, and it was mainly the time in rich conditions that determined the extent of the deactivation. The STEM images coupled with elemental analysis revealed the formation of large Cu particles during rich aging. Conversely, Cu remained well dispersed after lean aging. These results suggest that the copper migration and agglomeration in large extra-framework particles, accelerated by the action of hydrogen, caused the observed severe deactivation.