The storage of NOx under lean conditions has been studied for a Pt/K/Mn/Al2O3-CeO2 catalyst by operando FTIR spectroscopy. We have found that the first step of NOx adsorption is NO oxidation, NO2 being subsequently stored on potassium sites as nitrites; thereafter, nitrites are finally oxidized to nitrates by further NO2 molecules. It has been pointed out that the kinetic of these processes is strongly related to NO oxidation rate, both of them being slow at low temperatures, whereas they proceed faster at high temperatures. Two different periods have been characterized during NOx storage, a first one presenting full consumption of the nitrogen oxides, and a second one where NO and NO2 appear in the gas phase, to finally reach a steady level; the parameters governing the length of these periods are the progressive adsorption of NOx all along the sample and the presence of two different potassium sites, i.e. surface and bulk sites. Finally, the excellent storage properties of the catalyst studied here have been correlated with the presence of a particular phase, which has been characterized by transmission electron microscopy (TEM). (c) 2006 Elsevier B.V. All rights reserved.