Supported metal catalysts for wet decomposition of ammonium nitrate were investigated in order to develop the catalysts, which can convert both ammonium ion and nitrate ion selectively to N-2 and H2O under mild conditions, and to investigate the possible reaction pathways for simultaneous decomposition of ammonium and nitrate. Pt/active carbon (AC) catalysts were revealed to be the most active among the catalysts examined in this study. Effects of the reaction conditions such as metal loading, reaction temperature, and gas-feed pressure on the conversion and selectivity for N-2 were also investigated. 2 wt.% Pt/AC decomposed ammonium nitrate effectively under 0.5 MPa of air at 453 K showing the conversion of 99% or higher and N-2 selectivity of 95% or higher. While N2O was the main byproduct with selectivity of 5% or lower in the reaction using Pt/AC catalysts, CO2 was also formed at different yield depending on the activity, indicating AC support worked as a reducing agent, i.e. AC played an important role in formation of N-2 as well as in dispersion of the active metal as the support. NO2-, CO, NO, and NO2 were hardly formed. A series of experiments using isotope-labeled ammonium nitrate revealed that simultaneous removal of ammonium ion and nitrate ion did not take place via independent reactions of the two ions but via the equimolecular reaction between the two and that wet air oxidation of ammonium ion competed with the reaction of ammonium ion with nitrate ion. N2O was the major intermediate of the reaction for all the reactions studied. (C) 2004 Elsevier B.V. All rights reserved.