Laboratory-scale, continuous-flow reactor tests were conducted to confirm the destruction efficiency of methylphosphonic acid (MPA) and the effect of sodium hydroxide on MPA destruction efficiency under supercritical water oxidation (SCWO) conditions. Oxygen was used as the oxidant. The reaction temperatures ranged from 400 to 594 degrees C; the reactor residence times varied from 3 to 83 s; and the oxygen concentrations varied from 110 to 200% of stoichiometric requirements. Fixed parameters included (1) a nominal pressure of 27.6 MPa (4000 psi), (2) a MPA feed concentration of 1000 mg/L, (3) a feed flow rate of 25 g/min, and (4) a NaOH to MPA molar ratio of 2:1. MPA destruction efficiencies (DE) of greater than 99% were achieved at a temperature of 550 degrees C, oxygen concentration of 200% stoichiometric requirements, and reactor residence time of less than 20 s. On the basis of data derived from 43 MPA experiments, kinetic correlations for the DE of MPA were developed. The model predications agreed well with the experimental data. Furthermore, data derived from 22 MPA/NaOH experiments indicated that NaOH did not affect the overall effectiveness of SCWO for the destruction of MPA under the conditions investigated.