In this study, fresh and laboratory-aged (rapid ageing hot in laboratory (RAHLAB)) Pd-Ba-OSC/Al2O3 and Pd-OSC/Al2O3-La2O3 catalysts were investigated. The effect of low barium content on the behaviour of the OSC material (Ce1-xZrxO2 mixed oxides) was studied. The influence of the addition order of Ba, Pd, and OSC material in the catalyst washcoat was also examined. In a model reaction NO + CO, the catalytic activities and selectivities to N-2 and N2O were analysed by the FF-IR technique. In addition, the effect of thermal ageing treatment on catalytic behaviour was investigated. The addition of barium was found to have several positive effects on the fresh Pd-Ba-OSC/Al2O3 catalysts. The catalytic activity (NO reduction by CO) was improved below the light-off temperature when barium was added tnto the catalyst. At 500 degrees C, the fresh catalysts were fully selective to N2. The barium-containing catalysts had a bit higher dispersion values compared to the sample without barium. The high surface area over the fresh barium-containing catalysts could explain partly the improvements in the dynamic oxygen storage capacity (OSC). Besides, that the barium-containing catalysts were more active than the Pd/(OSC-La2O3-Al2O3) catalyst, it was also observed that the addition order of barium has an effect on the catalytic activity of fresh catalysts. The fresh (Pd-Ba-OSC)/Al2O3 and (Ba-Pd-OSC)/Al2O3 catalysts were more active than the Pd/(OSC-La2O3-Al2O3) catalyst. This indicates that the interaction of Pd, Ba, and OSC was better on these catalysts. For the aged catalysts, the addition order of Pd, Ba, and OSC had no obvious effect on differences in catalytic properties between Ba-containing samples. All the aged catalysts had high selectivities to N-2 at 500 degrees C. Characterisation results by XRD, chemisorption, and BET revealed that catalytic deactivation was occurring during the ageing process. This thermal deactivation was observed as a lower catalytic activity and a lower OSC compared to the fresh catalysts. The deactivation is due to the collapse of the surface area, sintering of I'd particles and solid-solid phase transitions observed in the bulk material. Therefore, Ba content (only similar to 3 wt.%) in the catalyst was probably too small to stabilise the catalyst against sintering. (c) 2005 Elsevier B.V. All rights reserved.