The selective reduction of nitrogen dioxide and nitrogen monoxide by olefins (ethene, propene) has been studied over two different gamma-aluminium oxides in the temperature range 473-873 K. Nitrogen dioxide was reduced more effectively than nitrogen monoxide with both, ethene and propene, as a reductant. At temperature exceeding 700 K, ammonia was formed as a by-product over one type of alumina. Concentrations in the range 30-40 ppm were determined for propene in combination with both, NO and NO2, while no ammonia was produced with ethene as a reductant. In addition, significant formation of hydrogen cyanide up to 70 ppm was observed with propene over both aluminium oxides starting from either NO or NO2. In contrast, hydrogen cyanide formation remained below 10 ppm with ethene as a reductant. Nitrous oxide formation did not exceed 10 ppm for all investigations. The results show that for alumina catalysts ethene is a more suitable reductant than propene due to its lower tendency to form undesired by-products.