Abstract: The present invention disclose a sulfur oxide trap that may be used to improve efficiency of a nitrogen oxide trap. Sulfur oxide has a deleterious effect on the performance of nitrogen oxide traps. The sulfur oxide trap of the present invention, comprises a monolithic substrate which is over-coated with an aluminum oxide layer and a mixed oxide layer of calcium oxide and magnesium oxide. In another embodiment of the invention, the sulfur oxide trap is integrated with a nitrogen oxide trap by coating the catalyst contained within a nitrogen oxide trap with a mixed oxide layer of calcium oxide and magnesium oxide. In each embodiment of the invention, the sulfur oxide trap can be regenerated by heating at elevated temperature for a short time period.