The selective catalytic reduction of nitrogen oxides in the presence of oxygen and water vapor was investigated over perovskite-type mixed oxides La(0.8)A(0.2)MnO(3) (A = Sr, Ba, K, Cs) and LaMn(0.8)A(0.2)MnO(3) (B = Ni, Zn, Cu) with propene as the reductant. in the temperature range of 150-550 degreesC. The catalysts were contacted with a reactant mixture containing 1000 ppm NO. 500 ppm C3H6, 5% O-2 and 5% H2O in helium at a space velocity of 14.4 cm(3)/g s. A maximum was observed in the conversion of nitrogen oxides to nitrogen with temperature in the range of 300-400 degreesC. In the absence of propene, the activity of these perovskites for NO removal in the 300-550 degreesC range increased from approximately 20% (with propene) to 40% (without propene). Water inhibited the NOx removal process as well, the conversion levels in the propene and water-free gas mixtures rising to about 50-55%. The removal of nitrogen oxides did not take place in the absence of oxygen. Two different reaction mechanisms were postulated: (i) the selective reduction with propene at 150-300 degreesC, (ii) the direct decomposition. which is inhibited by the presence of propene and water, at 300-550 degreesC. The nitrogen dioxide formed by the perovskite-catalyzed oxidation of nitrogen monoxide appears to play a key role in both mechanisms.