A thin self-supported wafer of a commercial NOx-Storage-Reduction (NSR) material was investigated by operando transmission FTIR to determine the nature of the surface and bulk species formed over the sample during the lean and rich cycles, while simultaneously monitoring the reaction cell effluent by online mass spectrometry and chemiluminescence. Conditions representatives of those found in catalytic converters were used, including large concentrations of water and CO2. Water appeared to have a complex effect on NSR activity: on the one hand it hindered NOx storage and, on the other hand, it facilitated NOx reduction. Our data showed that both ionic and covalent carbonates were replaced by ionic and covalent nitrates when NOx were present. The large NOx uptake observed was mostly related to the replacement of ionic carbonates by ionic nitrates, mostly in the barium carbonate phase - but not in the bulk of large barium carbonate particles. The nitration of the bulk carbonate sites always remained low, i.e. <10% in all cases. It is possible that some other elements present (such as Mg, Ce, Sr, Cs) also contributed to some extent to the formation of the ionic species. The contribution of covalent species (e.g. formed on surface sites of alumina and ceria) to the storage was negligible, though some of those species could potentially be intermediates in the surface migration of ionic species. The trapping on hydroxyl sites was found to be negligible in the present case. (C) 2014 Elsevier B.V. All rights reserved.