Oxidation and hydrolysis rates of the model organophosphorus compound, methylphosphonic acid (MPA), were measured in supercritical water. The effects of MPA concentration (0.5 to 1.0 mM), oxygen concentration (1.0 to 3.8 mM), temperature (478 to 572 degreesC) and pressure (138 to 277 bar) on oxidation rates were determined for residence times ranging from 3.0 to 9.5 s. The MPA oxidation rate was dependent on both oxygen concentration and pressure (or water density), but was relatively independent of initial MPA concentration. The only phosphorus-containing product was phosphoric acid, while the primary carbon-containing products included carbon monoxide, methane, and carbon dioxide. Data were regressed to an empirical global rate law with a pre-exponential factor of 10(14.0+/-1.6) (s(-1) M-1.47), an activation energy of 228+/-22 kJ/mol, a first-order MPA dependence, an oxygen order of 0.30+/-0.18, and a water order of 1.17+/-0.30 (all parameters to 95% confidence). A macroscopic model was also developed to estimate reaction rate constants and oxygen dependences of the major identified reaction pathways. (C) 2004 American Institute of Chemical Engineers.