Commercial Cu-exchanged small-pore SSZ-13 (Cu-SSZ-13) zeolite catalysts are highly active for the standard selective catalytic reduction (SCR) of NO with NH₃. However, their activity is unexpectedly inhibited in the presence of NO₂ at low temperatures. This is strikingly distinct from the NO₂-accelerated NO_x conversion over other typical SCR catalyst systems. Here, we combine kinetic experiments, in situ X-ray absorption spectroscopy, and density functional theory (DFT) calculations to obtain direct evidence that under reaction conditions, strong oxidation by NO₂ forces Cu ions to exist mainly as Cu^II species (fw-Cu²⁺ and NH₃-solvated Cu^II with high CNs), which impedes the mobility of Cu species. The SCR reaction occurring at these Cu^II sites with weak mobility shows a higher energy barrier than that of the standard SCR reaction on dynamic binuclear sites. Moreover, the NO₂-involved SCR reaction tends to occur at the Brønsted acid sites (BASs) rather than the Cu^II sites. This work clearly explains the strikingly distinctive selective catalytic behavior in this zeolite system.