Nitrogen oxide storage catalysts are used to remove nitrogen oxides from the lean exhaust gas of so-called lean engines. By being exposed to high temperatures during vehicle operation, said catalysts are subjected to thermal aging processes affecting nitrogen oxide storage catalysts and precious metals containing catalytically active components. The present invention relates to a method by means of which the catalytic activity of a nitrogen oxide storage catalyst lost as a result of the thermal aging process may be at least partially restored, containing as catalytically active compounds, in addition to platinum, alkaline compounds of strontium and/or barium on a carrier material containing ceroxide. The dual-stage method is based on the fact that the carrier material may be used to return strontium and/or barium compounds formed during thermal aging to the catalytically active form thereof by targeted treatment with special gas mixtures.