Cr–Mn mixed-oxide based catalysts were prepared for the low-temperature selective catalytic reduction of NO x with ammonia in the presence of excess oxygen. It was found that the Cr(0.4)–MnO x showed the highest activity and yielded 98.5% NO x conversion at 120°C. XRD, TPR and Raman data results suggested that a crystalline phase of CrMn 1.5 O 4 was present in the Cr–MnO x catalysts, which contained the active species. XPS results of fresh, used and regenerated Cr(0.4)–MnO x catalysts illustrated clearly the presence of Mn 2+ , Mn 3+ , Mn 4+ and Cr 2+ , Cr 3+ , Cr 5+ oxidation states. Efficient electron transfer between Cr and Mn in the crystal of CrMn 1.5 O 4 was thought to be the reason for the high activity and long lifetime of the Cr(0.4)–MnO x catalyst. In addition, the SCR activity was gradually suppressed in the presence of SO 2 , while such an effect was shown to be reversible after switching off the SO 2 injection.