The invention relates to a nitrogen oxide storage catalyst consisting of at least two catalytically active coatings on a supporting body, a lower coating (A) containing cerium oxide in addition to platinum and/or palladium and no alkaline earth compound and an upper coating (B), which is arranged above coating (A), containing an alkaline earth compound and a basic magnesium-aluminium mixed oxide in addition to platinum and palladium. The invention also relates to a method for converting NOx in the exhaust gases of motor vehicles operated using lean-burn engines.