Selective catalytic reduction of NO by propene was investigated on monometallic (Co or Ni) and bimetallic (Co/Ag or Ni/Ag) mordenite-based catalysts in the presence of an oxygen excess. Results showed that the catalytic activity of monometallic catalysts prepared from the zeolite sodium form was higher than that for the catalysts prepared from the sodium form. Opposite performances were found for the bimetallic (Co/Ag or Ni/Ag) catalysts. These results were related to both the location and stability of the metal cations, as well as the acid site density. From "in situ" IR results, a first approach to the possible mechanism has been proposed. Bimetallic samples in the acid form could keep up to 80% of the initial activity in the presence of 10% of water in the feed. SO2 inhibited the SCR activity of the catalysts at low temperature (< 400 degrees C), whereas it showed a promoting effect at temperatures higher than 400 degrees C.