Eight (8) perovskite-type mixed oxides having the general formula La1-xSrFeO3+/-delta (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 degreesC. As determined by Mossbauer spectroscopy the gradual substitution of La by Sr results in transformation of Fe3+ in the solids to Fe4+ existing in the perovskite structure SrFe4+ O-3. Other crystal phases detected by XRD include LaFeO3 and Fe2O3. 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 degreesC. The most active solids are the ones possessing the lowest amount of Fe4+ and the highest amount of Fe3+. The reaction rate for the NO and CO elimination decreases as the difference delta( - DeltaH(M-O)) = Sigma ( - DeltaH(M-O))(Fe3+,Fe4+) - Sigma( - DeltaH(M-O))(Fe3+) increases, where Sigma( - DeltaH(M-O))(Fei+) is the sum of enthalpies of the M-O bonds of the nominal solids containing Fei+ (i = 3, 4) iron species.