NH3 adsorption and desorption behavior of a commercial Cu-chabazite (CHA) NH3 selective catalytic reduction (NH3-SCR) catalyst was studied in the presence and absence of H2O. NH3 uptake values at various adsorption temperatures were obtained during various steps of the adsorption and temperature-programmed desorption (TPD) experiments. Total NH3 uptake decreased from 4.6 to 1.6 g NH3/L catalyst when the adsorption temperature was increased from 50 to 300 degrees C. Three major adsorption sites for NH(3)adsorption could be identified and quantified using TPD experiments, namely loosely, moderately, and strongly bound NH3 with peak centers at around 147, 266, and 447 degrees C. The total NH3 uptake was significantly affected by the presence of H2O in the feed. This resulted in a significant uptake loss (nearly 60%) for the loosely bound NH3. Three single-site and one three-site model were developed and compared in terms of NH3 uptake and release. The effects of site density values and thermodynamic restrictions in one-site models were investigated. The model using site density values obtained during the TPD phase resulted in the best fit among one-site models. The three-site model, which uses site density values obtained using dry adsorption of NH3, best represented the experimental data.