High-resolution XPS, including time- and temperature-dependent measurements, and temperature-programmed reactions, including O-18 isotope mixing studies, have been used to study the sulfoxy species formed on Ag-{100} in the absence and presence of Cs. On the alkali-free preoxygenated surface, SO2 adsorbs to form a surface sulfite which decomposes at similar to 460 K releasing gaseous SO2 and dissolved oxygen. Cs induces the formation of a more strongly bound alkali sulfite which reacts further to form alkali sulfate. The alkali sulfite and sulfate decompose at similar to 525 and similar to 575 K, respectively, releasing gaseous SO2, dissolved oxygen, and a small amount of adsorbed sulfur. Isotope mixing data indicate that the sulfoxy species formed in the presence of alkali have a higher bond order with the Ag surface than the sulfite formed on the alkali-free surface. All these species are labile with respect to oxygen release in the temperature regime relevant to oxyanion-promoted ethene epoxidation, and a reaction scheme is proposed that accounts for their formation, interconversion, and decomposition.