Fresh and SO₂-poisoned Cu-SSZ-13 selective catalytic reduction (SCR) catalysts were studied using near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) in the presence of NH₃, O₂, and NO, as well as by in situ X-ray absorption spectroscopy (XAS) using H₂, He, and CO. In contrast to the fresh catalyst, inhibited reduction of Cu-species and the absence of Cu-metal nanoparticles was found in the SO₂-poisoned catalyst during heating/cooling in H₂ and CO. High structural disorder and differences in the formation of Cu-carbonyl species were seen for the SO₂-poisoned catalyst compared to the fresh catalyst. Suppressed oxidation-reduction and low mobility of Cu-species during exposure to NH₃-SCR-related gases were observed in the SO₂-poisoned catalyst, unlike in the fresh catalyst. These observations help explain catalyst deactivation during the standard NH₃-SCR reaction. The formation of Cu-metal nanoparticles in the fresh catalyst revealed another possible deactivation pathway for the SCR-catalyst in combined LNT-SCR systems during fuel-rich periods.