Abstract: Process and system are described for control of sulfur oxide in emissions to the atmosphere from regenerators of Fluid Catalytic Cracking Units (FCC) in a manner which improves the operation of the regenerator and/or the reactor. Spent FCC cracking catalyst containing sulfur-bearing inactivating carbonaceous deposit is partially oxidized, preferably in the presence of steam, thereby producing a mixture of CO and CO.sub.2 and releasing sulfur in the resulting reducing atmosphere as vaporous hydrogen sulfide before the catalyst is regenerated, whereby the amount of sulfur convertible to noxious SOx in the regenerator flue gas is reduced. By the disclosed process and system, the quantity of oxygen introduced is predetermined and controlled in response to hydrogen content of the coke on the partially oxidized catalyst or an SOx analyzer associated with the regenerator can be used as a control tool.