NOx storage mechanism over a model NSR catalyst has been analysed by means of in-situ FTIR. The results indicated that a two-step mechanism involving nitrite formation, without requirement of NO evolution to NO2, followed by oxidation to nitrate species, being both steps assisted by O2, would describe the overall process at 350 °C. This mechanism could be also extended to a wider temperature range. The interaction between Pt and Ba sites was crucial in this mechanism, since spillover process of oxidising agents appeared to play a key role. NO2 direct interaction with BaO surface may also occur, but this process was only dominant on Ba sites away from Pt interaction.