The decomposition of aqueous ammonium nitrate at elevated temperatures and pressures is examined as a function of chloride, nitrate, and total acidity. Catalysis requiring both chloride and acid was observed in solutions containing 20% (w/w) NH4NO3 at 180 degrees C. Nitrous oxide and dinitrogen were generated in a 4:1 ratio below 0.2 M H+. Dinitrogen formation correlated with the production of additional acidity by the reaction 5NH(4)NO(3) --> 4N(2) + 2HNO(3) + 9H(2)O. The second-order dependence of the decomposition reaction on [H+] is consistent with the reversible formation of NO2+. Incorporation of O-18 into the N2O product, as well as the inverse deuterium isotope effect, supports this conclusion. A novel mechanism based on the intermediacy of NO2Cl is proposed for the chloride catalysis and contrasted to the radical-based pathways operational in molten NH4NO3 decompositions. Isotope-labeling experiments using (NH4NO3)-N-15 lead to the formation of N-15=N=O-labeled nitrous oxide and the dinitrogen products N-15=N-15 and N=N-15 in a 1:3 ratio. Decomposition of (NH4NO3)-N-15 produces only N=N-15=O and N=N-15. This agrees with the reaction sequences: NO2Cl + NH4+ --> {O2N-NH3}(+) + H+ + Cl-, {O2N-NH3}(+) --> N2O + H3O+ and 3NO(2)Cl + 3NH(3) --> 3NH(2)Cl + 3NO(2)(-) + 3H(+), 3NH(2)Cl --> N-2 + NH4+ + 3Cl(-) + 2H(+), 3NO(2)(-) + 3NH(4)(+) --> 3N(2) + 6H(2)O. These results bear on the industrial preparation of NH4NO3 and suggest conditions under which nitrous oxide emissions might be important to the global N2O budget.