The interaction of sulfur dioxide with a commercial NOx storage-reduction catalyst (NSR) has been investigated using in situ IR and X-ray absorption spectroscopy. Two pathways of catalyst deactivation by SO2 were identified. Under lean conditions (exposure to SO2 and O-2) at 350 C the storage component forms barium sulfates, which transform from surface to hardly reducible bulk sulfate species. The irreversible blocking of the Ba sites led to a decrease in NOx storage capacity. Under fuel rich conditions (SO2/C3H6) at 350-500degreesC evidence for the formation of sulfides on the oxidation/reduction component (Pt) of the catalyst was found, which blocks the metal surface and thus hinders the further reduction of the sulfides.