A combination process is provided in which a gaseous mixture comprising SO.sub.x and NO.sub.x is reacted with cerium oxide at a temperature ranging from about 500.degree. C. to about 700.degree. C. to form cerium oxysulfate and a gas which is at least partially desulfurized. The latter is contacted with cerium oxysulfate at a temperature ranging from about 500.degree. C. to about 700.degree. C. in the presence of ammonia at net oxidizing conditions to reduce the NO.sub.x content of the gaseous mixture. The process may be conducted in one reaction zone or in a plurality of reaction zones. The cerium oxide reactant may be regenerated by reacting cerium oxysulfate with a reducing gas such as hydrogen.