The selective catalytic reduction of NOx (NO + NO2) at low temperature with ammonia has been investigated with natural manganese ore, pure manganese dioxide, and manganese dioxide supported alumina. The catalysts showed high activities for NOx reduction with NH3 in the presence Of O-2 at temperatures below 250 degreesC. The decrease of SCR activity without oxygen in the gas phase differed among the three catalysts in accord with their differing amounts of lattice oxygen. The SCR activity of the catalysts, in that case, decreased in the order MnO2 > NMO > MnO2/gamma -Al2O3. Small amounts Of SO2 deactivated the catalysts in the low-temperature range. An XRD analysis of the sulfated catalysts with differing temperature provides evidence that the formation of ammonium sulfates is the main poisoning route. These results could restrict the practical application of these catalysts to sulfur-free conditions.