The influence of the acid property of solid acids on the selective reduction of nitrogen monoxide by catalyst composites from perovskite and solid acid materials was examined for the case of excess of oxygen by using the perovskite YFeO3 and the solid acids TiO2 and ZrO2. For comparison cordierite was likewise employed as support material. It is shown that the catalytic activity of the composite strongly depends on the acid properties of the solid acid : the activity is higher for materials with stronger acidity. For composites YFeO3/ZrO2 the SCR activity starts around 250 degrees C. Remarkably, the conversion of NO is completely selective to N-2 and no dinitrogen monoxide is produced. Impregnation by the noble metals Pt and Ag is without influence on the SCR activity. A pronounced improvement of the lean-NOx activity is obtained for mixtures with the Cu exchanged zeolite Cu/ZSM-5.