The Co,Ba,K/CeO₂ system is a potential candidate to be used under the simultaneous abatement of soot and NO_x in diesel exhausts. In this work, we study the interactions of NO_x 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 490°C. However, when Co is incorporated to the system (Co,Ba,K/CeO₂ catalyst), Ba nitrates become unstable at ca. 370°C, under both NO+O₂ and He atmosphere. DRX, FTIR, and LRS characterization shows that potassium favors the formation of the BaCoO_2.93 perovskite in the Co,Ba,K/CeO₂ catalyst calcined at 400°C. This perovskite structure is not detected when K is absent (Co,Ba/CeO₂ system). FTIR spectra of Co,Ba,K/CeO₂ treated with NO+O₂ 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 BaCoO_2.93, 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 N₂ under a reducing atmosphere, making this system very interesting to be used as a NO_x catalytic trap. On the other hand, under He atmosphere the decomposition of nitrates only produces gaseous NO_x. In addition, when the Co,Ba,K/CeO₂ catalyst previously treated under NO+O₂ atmosphere is intimately mixed with the soot and then heated in He up to 490°C, the soot particulates are partially oxidized by the surface nitrate species.