The Co,Ba,K/CeO2 system is a potential candidate to be used under the simultaneous abatement of soot and NOx in diesel exhausts. In this work, we study the interactions of NOx with the said solid. NO2 is strongly adsorbed on the Ba,K/CeO2 surface through the formation of Ba and K nitrate species which are stable under He atmosphere up to 490degreesC. However, when Co is incorporated to the system (Co,Ba,K/CeO2 catalyst), Ba nitrates become unstable at ca. 370degreesC, under both NO + O-2 and He atmosphere. DRX, FTIR, and LRS characterization shows that potassium favors the formation of the BaCoO2.93 perovskite in the Co,Ba,K/CeO2 catalyst calcined at 400degreesC. This perovskite structure is not detected when K is absent (Co,Ba/CeO2 system). FTIR spectra of Co,Ba,K/CeO2 treated with NO + O-2 indicate the formation of surface N-bound nitrate species (O-Ba- -NO2), where NO2 acts as a Lewis base. Since the said species are not detected in the samples without BaCoO2.93, it is suggested that they are related to the Ba atoms of the perovskite structure. Microbalance experiments combined with FTIR and mass spectroscopy characterization show that the nitrate species formed in the Co,Ba,K/CeO2 sample can be readily decomposed to N-2 under a reducing atmosphere, making this system very interesting to be used as a NOx catalytic trap. On the other hand, under He atmosphere the decomposition of nitrates only produces gaseous NOx. In addition, when the Co,Ba,K/CeO2 catalyst previously treated under NO + O-2 atmosphere is intimately mixed with the soot and then heated in He up to 490degreesC, the soot particulates are partially oxidized by the surface nitrate species. (C) 2003 Elsevier Inc. All rights reserved.