A supercritical water co-oxidation elementary reaction rate mechanism was constructed from submechanisms for methylphosphonic acid (MPA or PO(OH)(2)CH3) and ethanol with updated kinetic rate parameters for H,H2O2 and HOCO center dot chemistry. The co-oxidation mechanism accurately reproduces the experimentally observed conversion trend of the refractory MPA component as a function of initial concentration of the labile ethanol component [J.M. Ploeger, P.A. Bielenberg, R.P. Lachance, J.W. Tester, Co-oxidation of methylphosphonic acid and ethanol in supercritical water: I.. Experimental results, J. Supererit. Fluids (2006)]. The increase in MPA conversion with increasing ethanol concentration is predicted to be caused by the increased concentration of hydroperoxy radicals (HO2 center dot) produced by ethanol oxidation. An analysis of the major organophosphorus reaction fluxes indicated that the co-oxidative effect would increase the conversion of MPA but not change the rate of formation of methane. An experiment using a model formaldehyde/methanol mixture as a co-oxidant was conducted to confirm this prediction. (c) 2006 Elsevier B.V. All rights reserved.