Reaction studies have been carried out on the hydrothermal oxidation of ammonium using nitrate as the oxidizer. Raman spectroscopy was used to make in situ measurements of reactant and product species. Reaction rates were determined from concentrations derived from Raman spectra and from measurements of system pressure. Concentrations of key species in the reactor effluent were determined via off-line assay. The effects of temperature, pressure, pH, and nitrate-to-ammonium molar ratio on reaction rate were determined. Oxides of nitrogen were not generated in quantities detectable by Raman spectroscopy. The reaction half-life is below ten seconds at 673 K with better than 95% conversion. The studied system is a model for the destruction of ammonium in municipal wastes.