The dynamic character of the active centers has made it difficult to unravel the reaction path for NH3-assisted selective catalytic reduction (SCR) of nitrogen oxides over Cu-CHA. Herein, we use density functional theory calculations to suggest a complete reaction mechanism for low-temperature NH3-SCR. The reaction is found to proceed in a multisite fashion over ammonia-solvated Cu cations Cu­(NH3)2 + and Brønsted acid sites. The activation of oxygen and the formation of the key intermediates HONO and H2NNO occur on the Cu sites, whereas the Brønsted acid sites facilitate the decomposition of HONO and H2NNO to N2 and H2O. The activation and reaction of NO is found to proceed via the formation of nitrosonium (NO+) or nitrite (NO2 –) intermediates. These low-temperature mechanisms take the dynamic character of Cu sites into account where oxygen activation requires pairs of Cu­(NH3)2 + complexes, whereas HO–NO and H3N–NO coupling may occur on single complexes. The formation and separation of Cu pairs is assisted by NH3 solvation. The complete reaction mechanism is consistent with measured kinetic data and provides a solid basis for future improvements of the low-temperature NH3-SCR reaction.