Low loaded alumina supported manganese oxides exhibit a high activity and selectivity for the selective catalytic reduction (SCR) of NO in the temperature range 383-623 K. The impact of low concentrations of SO2 on the activity of these catalysts has been investigated. Upon SO2 addition to the flue gas, the catalysts lose their high initial activity in a few hours due to stoichiometric SO2 uptake. Analysis of the deactivated samples by mercury porosimetry, FTIR, TPR and TPD shows that the deactivation is not due to the formation of (bulk or surface) Al-2(SO4)(3) or deposition of ammonium sulphates. Comparison of the results with unsupported Mn2O3 and MnO2 provides evidence that formation of surface MnSO4 is the main deactivation route. This process is independent of the oxidation state of the manganese and the presence of oxygen in the gas stream. The formed sulphates decompose at 1020 K and are reduced by H-2 at temperatures above 810 K. This means that regeneration of the catalysts is not very feasible. The results restrict practical application of these catalysts to sulphur free conditions.