Selective catalytic reduction of nitric oxide by ammonia (NH₃-SCR) is a promising technology for NO_x emission control. In this work, the recently discovered sulfate promotion effect in this reaction was thoroughly investigated on ceria. Sulfates derived from different organic S precursors all exhibit a promotion effect. Mechanistic studies generate a reaction network including both Langmuir–Hinshelwood and Eley–Rideal mechanisms. Sulfates were shown to be versatile promoters with multiple functions: they tune the adsorption of reactants towards a more balanced intermediate coverage by creating strong Lewis acid sites, facilitate *NH₂ formation, and suppress ammonia oxidation through surface oxygen deactivation. The comparison between sulfates and the classic inorganic promoter FeO_x revealed that sulfates are more effective, especially at high temperature. This is mainly attributed to the high coverages of Eley–Rideal intermediates and low ammonia oxidation activity on sulfated ceria. This work provides fundamental understanding of the role of surface sulfates in NH₃-SCR, which are essential under actual operation conditions. This understanding presents an exciting opportunity for future catalyst design with high durability and low cost.