Active centers in Cu/SSZ-13 selective catalytic reduction (SCR) catalysts have been recently identified as isolated Cu2+ and [Cu-II(OH)](+) ions. A redox reaction mechanism has also been established, where Cu ions cycle between Cu-I and Cu-II oxidation states during SCR reaction. While the mechanism for the reduction half-cycle (Cu-II -> Cu-I) is reasonably well-understood, that for the oxidation half-cycle (Cu-I > Cu-II) remains an unsettled debate. Herein we report detailed reaction kinetics on low-temperature standard NH3SCR, supplemented by DFT calculations, as strong evidence that the low-temperature oxidation half-cycle occurs with the participation of two isolated Cu' ions via formation of a transient [CuI(NH3)(2)](+)-O-2 [CuI(NH3)(2)](+) intermediate. The feasibility of this reaction mechanism is confirmed from DFT calculations, and the simulated energy barrier and rate constants are consistent with experimental findings. Significantly, the low-temperature standard SCR mechanism proposed here provides full consistency with low-temperature SCR kinetics.