The catalytic activity of YFeO3-based catalysts in the lean-NOx process was studied for the sulfated perovskite YFeO3, composites between YFeO3 and the solid acid ZrO2, as well as sulfated composites YFeO3/ZrO2. Moreover, a mechanical mixture between YFeO3 and sulfated tetragonal zirconia (t-ZrO2/SO4) was studied. It is shown that the lean-NOx activity of YFeO3 is improved by forming the composites YFeO3/ZrO2 as well as by sulfating samples, with the highest conversion rates of NOx into N-2 being about 30% at about 400 degrees C. The disadvantage of these materials is that the activity decreases with increasing the oxygen content of the feed gas. However, the mechanical mixture between YFeO3 and t-ZrO2/SO4 shows not only an improved high-temperature activity at about 400 degrees C but also a pronounced low temperature (<300-350 degrees C) activity. The latter clearly increases with increasing oxygen content, whereas the former decreases. The opposing effect of oxygen is explained by different reaction mechanisms.