The effect of silver on the activity and hydrothermal stability of Cu-ZSM-5 for NOx reduction with C3H8 under oxidizing and wet conditions has been investigated. The addition of silver as a cocation to Cu-ZSM-5 catalysts decreases the reversible inhibition by water occurring under catalytic conditions when the temperature does not exceed 773 K. Furthermore, it suppresses the irreversible deactivation observed when the reaction is performed up to 873 K. In effect, the Cu-Ag-ZSM-5 solids are more resistant to the dealumination induced by the acids formed by reactions between NO, O-2 and H2O than the Cu-ZSM-5 ones. The presence of silver provides also an higher hydrothermal stability under an (air + 10 vol% H2O) mixture if the temperature does not exceed 973 K. The catalytic activity and the deactivation behaviour are dependent on the concentration of silver and copper and more especially on the preparation procedure.