NH3 stored on zeolites in the form of NH4+ ions easily reacts with NO to N-2 in the presence of O-2 at temperatures <373 K under dry conditions. Wet conditions require a modification of the catalyst system. It is shown that MnO2 deposited on the external surface of zeolite Y by precipitation considerably enhances the NOx conversion by zeolite fixed NH4+ ions in the presence of water at 400-430 K. Particle-size analysis, temperature-programmed reduction, textural characterization, chemical analysis, ESR and XRD gave a subtle picture of the MnO2 phase structure. The MnO2 is a non-stoichiometric, amorphous phase that contains minor amounts of Mn2+ ions. It loses O-2 upon inert heating up to 873 K, but does not crystallize or sinter. The phase is reducible by H-2 in two stages via intermediate formation of Mn3O4. The manufacture of extrudates preserving stored NH4+ ions for NOx reduction is described. It was found that MnO2 can oxidize NO by bulk oxygen. This enables the reduction of NO to N-2 by the zeolitic NH4+ ions without gas-phase oxygen for limited time periods. The composite catalyst retains storage capacity for both, oxygen and NH4+ ions despite the presence of moisture and allows short-term reduction of NO without gaseous O-2 or additional reductants. The catalyst is likewise suitable for steady-state DeNO(x) operation at higher space velocities if gaseous NH3 is permanently supplied. (C)1999 Elsevier Science B.V. All rights reserved.