During recent years, catalytic combustion of low heating value gases has received increased attention. The purpose of the present work was to study the effect of ageing for 30 days at 1000 degrees C in air saturated with 12% steam on Pd- and Pt-impregnated as well as Mn-substituted lanthanum hexaaluminate materials. Both hexaaluminate powders and 400 cpsi cordierite monoliths, washcoated with hexaaluminate powder, were aged. Powders were characterised by BET and XRD, whereas the catalytic activity of the washcoated monoliths was evaluated in a bench-scale rig for conversion of synthetic gasified biomass. The surface areas decreased significantly during the first day of ageing, whereas further ageing had only a minor influence. The pure lanthanum-alumina sample was a mixture of the hexaaluminate LaAl11O18 phase and the less preferable perovskite LaAlO3 phase, which increased after ageing. The Mn-substituted lanthanum-alumina mainly showed pure hexaaluminate phase both before and after ageing. The catalytic activity tests showed that Pd-impregnated lanthanum hexaaluminate was the most active catalyst for combustion of carbon monoxide and hydrogen, retaining low light-off temperatures also after 30 days of ageing. However, the ignition temperature for 50% conversion (T-50) of methane was approximately 300 degrees C higher than for the fresh sample. Pt-impregnated samples were less active than the Pd ones. The Pt-loading decreased after ageing, whereas the Pd-loading remained fairly constant. However, the amount of Pd oxide decreased after ageing. Further, the Mn-substituted samples were less active than the precious metal ones. Here, the activity for the combustion of carbon monoxide was substantially affected by ageing. The formation of nitrogen oxides from ammonia was lower over the aged samples than over fresh ones; the Mn-substituted sample aged 30 days showed the lowest yield, only 30% of ammonia was converted to nitrogen oxides. (C) 2000 Elsevier Science B.V. All rights reserved.