Deactivation of catalysts based on either manganese oxides, copper oxides, platinum, palladium or combinations of these metal oxides and noble metals supported on gamma-alumina was studied. The activity of the catalysts for the oxidation of carbon monoxide, naphthalene and methane, in a mixture resembling the flue gases from wood combustion, was measured before and after exposure of the catalysts either to a temperature of 900 degrees C in the presence of steam or to sulphur dioxide. Most of the mixed catalysts were more resistant to hydrothermal and sulphur treatments than the catalysts with a single active component. After the hydrothermal treatment the activity of the MnOx catalyst was enhanced. When Pt is combined with MnOx or CuOx, the loss of activity of Pt was decreased during the hydrothermal treatment. Also, the hydrotreated mixed MnOx-Pd and CuOx-Pd catalysts were more active than the treated Pd catalyst for the oxidation of methane. After sulphur treatment, the activities of the mixed MnOx-Pt (Pt: 0.05 mol%), MnOx-Pd and CuOx-Pd catalysts were improved for the oxidation of carbon monoxide and naphthalene. Among the catalysts studied, the MnOx-Pt, CuOx-Pt and CuOx-Pd catalysts, with a metal oxide and a noble metal loading of 10 and 0.1 mol%/gamma-alumina, respectively had the best combination of activity, thermal stability and resistance to sulphur treatment.