The BaCoO_3−y perovskite-type mixed oxide is studied for both trapping of NO_x and combustion of diesel soot. The starting material consists of a stoichiometric mixture of Ba and Co nitrates. Different crystalline phases are obtained when the solids are calcined at different temperatures (ranging from 400 to 1000°C). After 400°C calcination Ba(NO₃)₂ and Co₃O₄ crystalline phases are formed, while after calcination at 700°C the BaCoO₃ stoichiometric perovskite is obtained. However, when the temperature is increased to 1000°C, the structure loses oxygen and perovskite BaCoO_2.74 is formed. The solids calcined at 700 and 1000°C show high NO_x adsorption capacity, the latter being more effective. This solid shows weak IR bands in the 800cm⁻¹ frequency region associated with the perovskite structure. After the catalyst interacts with NO_x, new bands associated with bulk nitrates and surface NO₃⁻ species are observed. Signals associated with surface N-bounded species of the OBaNO₂ type could be masked by the intense Ba(NO₃)₂ signals. LRS characterization is in agreement with XRD and FTIR results. The Raman signal at 716cm⁻¹ is associated with the BaCoO_2.74 structure while a broad signal at 607cm⁻¹ appears in samples containing BaCoO₃ and BaCoO_2.94 phases. Nitrates formed upon NO+O₂ treatments show high thermal stability under He atmosphere up to 490°C. However, reductive treatments either under H₂ atmosphere or with soot particles cause decomposition of the nitrates at temperatures lower than 400°C. A reaction scheme is proposed involving the participation of perovskite structures, Co₃O₄, Ba(NO₃)₂, BaO and metallic Co particles. The catalyst under study favors the reaction between the soot particles and the trapped NO_x species making this system promising for the simultaneous abatement of both contaminants. The addition of K decreases the soot combustion temperature.