In this study, the coordination sphere of copper in Cu–SSZ-13 as a catalyst for the selective catalytic reduction of NOx by ammonia is analyzed as a function of environmental parameters: temperature, partial pressure of water P(H2O), and partial pressure of ammonia P(NH3). By periodic density functional theory calculations, we obtain stability domains for variable loadings of water and ammonia (nH2O + mNH3 with (m + n ≤ 6)) close to CuII ions, which are located at 6-membered ring (6MR) or 8-membered ring (8MR) of the zeolitic structure. Ab initio calculations and thermodynamic investigations were performed to build phase diagrams, with vibrational analysis, so as to provide Gibbs free energy, G, values. Copper located in the 8MR appears to be more reactive toward H2O and NH3 adsorption than the one in the 6MR because of a lower coordination number of copper at 8MR in the absence of adsorbates. Depending on the operating conditions, structures containing adsorbed water and ammonia as ligands at the metal site can simultaneously be stabilized. The most widespread coordination number of CuII is 4 even at m + n > 4. The theoretical predictions were validated by in situ X-ray absorption spectroscopy, in dehydration conditions and in two gas atmospheres: dry He with P(NH3) = 10–3 bar (1000 ppm) and He with P(NH3) = 10–3 bar and P(H2O) = 10–2 bar. Trends in terms of ammonia desorption temperature as well as coordination numbers are well reproduced. Experimentally determined behaviors of CuI and CuII open new perspectives for the systematic computational investigation of the behavior of CuI in a H2O/NH3 atmosphere.