Modern diesel emission control systems often use Urea Selective Catalytic Reduction (Urea-SCR) for NOx control. One of the most active SCR catalysts is based on Cu-zeolite, specifically Cu-Chabazite (Cu-CHA), also known as Cu-SSZ-13. The Cu-SCR catalyst exhibits high NOx control performance and has a high thermal durability. However, its catalytic performance deteriorates upon long-term exposure to sulfur. This work describes our efforts to investigate the detailed mechanism of poisoning of the catalyst by sulfur, the optimum conditions required for de-sulfation, and the recovery of catalytic activity. Density functional theory (DFT) calculations were performed to locate the sulfur adsorption site within the Cu-zeolite structure. Analytical characterization of the sulfur-poisoned catalyst was performed using Extreme Ultraviolet Photoelectron Spectroscopy (EUPS) and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). Using these techniques, it was found that sulfur adsorbs on the ion-exchanged Cu sites in the zeolite structure. Since it is the Cu site within the zeolite structure that is responsible for the catalytic activity, blocking of this site by sulfur adsorption leads to a decrease in catalytic performance. To study the de-sulfation process, a systematic investigation into the effect of temperature, atmosphere, and SV (space velocity) was conducted. The Ea (activation energy) was determined for each reaction condition. Higher de-sulfation was observed at lower temperatures in the presence of NH3 as the reducing agent.