Abstract The activity and stability of Ba,K,Co/CeO2 catalyst in the diesel soot oxidation reaction are studied. Different modes of preparation are analyzed, varying both the cobalt precursor and its order of impregnation. In order to study the stability in a diesel exhaust atmosphere, the catalysts were pretreated in streams of CO2, H2O, NO and SO2. The fresh and the treated catalysts were characterized by FTIR and XRD techniques. The catalytic activity was measured by TPO of soot–catalyst mixtures. The tight contact was used to analyze the intrinsic activity. It was found that the activity was higher for the catalysts prepared using Co(NO3)2 compared with the catalysts prepared using Co(AcO)2. This is because in the former, KNO3 is present on the catalyst, being this compound very active for this reaction. The thermal stability is lower for the catalyst prepared with Co(NO3)2. This catalyst displays a higher K lost when treated at high temperature. When K is present as K2CO3, as is the case of the catalysts prepared with Co(AcO)2, the thermal stability is higher since K2CO3 is less volatile than KNO3. All the catalysts are stable in the presence of mixtures of (CO2 + H2O + NO + O2) having a composition similar to a real diesel exhaust. Under these conditions, the K mantains its original chemical state, either carbonate or nitrate, depending on the precursor used in the catalyst preparation. In presence of SO2, all the catalysts deactivate due to K2SO4 formation, which is not active for soot combustion. However, the sensitivity to SO2 depends on the precursors used to prepare the catalyst. The Ba,K,Co/CeO2 catalyst prepared impregnating Co(NO3)2 on the Ba,K/CeO2 catalyst has a higher resistance to the deactivation by SO2, since the following reaction occurs: Ba ( NO 3 ) 2 + K 2 SO 4 → BaSO 4 + KNO 3 , which implies that active KNO3 will dissapear slower from the catalytic surface, thus maintaining the activity. TPO experiments of the catalyst in loose contact with soot were also carried out. The Ba,K,Co/CeO2 catalyst prepared impregnating Co(NO3)2 on the Ba,K/CeO2 catalyst showed the higher activity in loose contact mode.