Cu-SSZ-13 and Cu-SSZ-39, with similar structures, are both highly active and hydrothermally stable in the selective catalytic reduction of NO_x with NH3 (NH3-SCR), attracting great attention for applications on diesel vehicles. In this study, it was interestingly found that NO2 has distinct effects on the NOx conversion over Cu-SSZ-13 and Cu-SSZ-39, with an inhibiting effect for Cu-SSZ-13 but a promoting effect for Cu-SSZ-39. The distinct NO2 effects were found to be associated with the differences in the reactivity of surface NH4NO3, a key intermediate for NH₃-SCR, on these two Cu-based small-pore zeolites. Cu-SSZ-13 has excellent standard SCR activity, but the reactivity of surface NH₄NO₃ with NO is relatively low, which would induce the accumulation of NH₄NO₃ on the surface and thus inhibit NO_x conversion. Surface Bronsted acid sites play key roles in the reduction of surface NH₄NO₃ by NO, and Cu-SSZ-39 showed much higher surface acidity than Cu-SSZ-13. Compared with Cu-SSZ-13, the intrinsic standard SCR activity of Cu-SSZ-39 was lower but NH₄NO₃ could be reduced by NO rapidly on Cu-SSZ-39, even faster than the reduction of NO by the adsorbed NH₃ on Cu active sites; thus, NO_x conversion was promoted by NO2 on Cu-SSZ-39. This work provides an improved understanding of fast SCR on Cu-based small-pore zeolites.