The role of methanol (MeOH) in the supercritical water oxidation (SCWO) of ammonia (NH3) was investigated near the typical operating conditions of a commercial plant at 560-620 degrees C and 25 MPa using a tubular reactor. It was found that the presence of MeOH strongly affected the reaction behavior of nitrogen. With the addition of MeOH, at concentrations more than twice the initial concentrations of NH3 (1.4-5.7 mmol/mol of the feed mixture), the conversion of NH3 to N2O increased by about four times to 0.5-0.6 compared to that with no MeOH addition. The production of NO3- from NH3 increased in the liquid effluent with a conversion of up to 0.2. The productions of NO2-, NO, and NO2 from NH3 had similar behavior under the presence of MeOH at the tested conditions, and significant effects were observed at MeOH concentrations more than 2-6 times of the initial concentrations of NH3. The experimental results made it clear that NO and NO2 were produced under the SCWO conditions. Methods for the reduction of N2O and NO3- were also investigated. Results showed that SCWO is not suitable for the decomposition of N2O. Other post-treatment methods such as catalytic decomposition should be applied to reduce N2O in the gas effluent. NO3- reduction methods (e.g., two-step reaction) proposed in this study should also be considered to prevent corrosion of the reactor material.