Cu-CHA materials are efficient catalysts for NH3-SCR of NOx in oxygen excess. A crucial step in the reaction is oxygen (O-2) activation, which still is not well understood. Density functional theory calculations in combination with ab initio thermodynamics and molecular dynamics are here used to study O-2 dissociation on Cu(NH3)(2)(+) species, which are present under NH3-SCR conditions. Direct dissociation of O-2 is found to be facile over a pair of Cu(NH3)(2)(+) complexes whereas dissociation on a single Cu(NH3)(2)(+) species is unlikely due to a high activation energy. The presence of NO promotes oxygen dissociation on both single and pairs of Cu(NH3)(2)(+) complexes. Nitrites and nitrates are easily formed as O-2 dissociates, and NO adsorption over nitrates leads to facile formation of NO2. The results stress the importance of ligand-stabilized Cu species in Cu-CHA catalysts for NH3-SCR. 2017 Elsevier Inc. All rights reserved.