The effectiveness of multistage methanol injection in supercritical water oxidation (SCWO) of an ammonia/methanol mixture as a method for utilizing the alcohol co-oxidation effect was studied at 530 degrees C and 25 MPa. A simple global reaction rate model was developed based on the results of kinetic experiments performed at various ammonia, methanol, and oxygen concentrations, yielding excellent agreement with experimental values. Ammonia conversion was predicted when the same molar flow rate of methanol was split between two points: a first injection at the entrance of a reactor and a second injection midway in the reaction stream. Two-stage methanol injection resulted in higher ammonia conversion than in the model calculation when methanol was injected once at the reactor entrance. Model calculations under the same experimental conditions showed that two-stage injection improves ammonia conversion, which aligns closely with predicted values. Moreover, our calculations suggest that ammonia conversion depends on injection timing and on methanol concentration of the flow after injection as well.