Eight (8) perovskite-type mixed oxides having the general formula La 1 - x Sr x FeO 3 + /- δ (x=0, 0.15, 0.3, 0.4, 0.6, 0.7, 0.8 and 0.9) were prepared using the corresponding nitrate salts and heating at 1000 o C. As determined by Mossbauer spectroscopy the gradual substitution of La by Sr results in transformation of Fe 3 + in the solids to Fe 4 + existing in the perovskite structure SrFe 4 + O 3 . Other crystal phases detected by XRD include LaFeO 3 and Fe 2 O 3 . The solids were tested for their catalytic activity for the reaction NO+CO->products in a lab-scale plug flow reactor (PFR) between 250 and 580 o C. The most active solids are the ones possessing the lowest amount of Fe 4 + and the highest amount of Fe 3 + . The reaction rate for the NO and CO elimination decreases as the difference δ(-ΔH M - O )= (-ΔH M - O ) F e 3 + , F e 4 + - (-ΔH M - O ) F e 3 + increases, where (-ΔH M - O ) F e i + is the sum of enthalpies of the M-O bonds of the nominal solids containing Fe i + (i=3, 4) iron species.