Temperature programmed desorption, infrared spectroscopy, and N-15 solid state NMR spectroscopy were used to characterize the nature of the nitrate species formed on Al2O3 and BaO/Al2O3 NOx storage/reduction materials. Two distinctly different nitrate species were found: surface nitrates that are associated with a monolayer BaO on the alumina support, and a bulk-like nitrate that forms on this thin BaO layer. The surface nitrates desorb as NO2 at lower temperatures than do the bulk-like nitrates, which decompose as NO+O-2 at higher temperatures. The amount of NOx stored in the monolayer nitrate is proportional to the surface area of the catalyst, while that in the bulk nitrate increases with BaO coverage.