Single phase perovskites, such as LaCoO₃ and La_1−xSr_xCoO₃, exhibit catalytic activity only in oxidation reactions. However, a mixture of La_1−xSr_xCoO₃, SrCoO_x and Co₃O₄ oxides presents both oxidation and reduction properties in the DeNO_x reaction. To improve DeNO_x activity, we have added 1–2 wt% of Ag to a mixture of nanosized oxides of La, Sr and Co. These mixtures (with and without Ag) were prepared by the sol–gel method using citric acid as the solvent. Two kinds of mixtures, A=La_1−xSr_xCoO₃+Co₃O₄+SrCoO_x and B=A+Ag, were characterized by x-ray diffraction (XRD) analysis, field emission scanning electron microscope (FESEM) and energy dispersive x-ray spectroscopy (EDS). Their catalytic activity was studied by temperature programmed surface reaction (TPSR). The results showed that (i) for catalyst A, the total C₃H₆-conversion temperature is in the range 210–500 °C, while for catalyst B that is 200–400 °C; (ii) the mixed oxide A exhibits a weak DeNO_x activity, while the silver doping leads to a remarkable increase in the NO_x-conversion from 29.4% (in the absence of Ag) to 82.4% at low temperature 220 °C in the lean burn condition.