Methylamine was oxidized in supercritical water in a Hastelloy tubular flow reactor at 249 atm and temperatures between 390 and 500 degrees C. The major carbon-containing products were CO2 and CO, with trace amounts of CH3OH. The major nitrogen-containing products were NH3, N2O, and N-2. A reaction network consistent with all the results has been constructed. Ammonia appears to be the exclusive nitrogen-containing intermediate between methylamine and the final products, N2O and N-2. The disappearance of ammonia during methylamine SCWO is markedly faster than that during oxidation of ammonia alone. Approximately 3-4 times more N2O is produced than N-2, whereas the N2O/N-2 ratio is essentially zero when ammonia is oxidized alone in SCW. We attribute these differences in the rate of and selectivity from ammonia oxidation in supercritical water in the present experiments to the presence of methylamine in the reaction environment and catalytic chemistry on the reactor walls.