We have examined the reactivity of methylamine in supercritical water (SCW) at temperatures between 386 and 500degreesC. The major products measured are ammonia and methanol. Batch reactor studies revealed a global reaction order of 0.66 +/- 0.11 for methylamine disappearance. For water densities less than 0.28 g/cm(3) and pressures less than or equal to 250 atm, the effect of water on the reaction rate appears to be negligible, and there is little evidence of hydrolysis (very little methanol formed). Under these conditions, most typical of supercritical water oxidation (SCWO) reactors, the reaction seems to be governed by pyrolysis. The rate constant for methylamine disappearance in this region is described by an activation energy of 38 +/- 7 kcal/mol and a pre-exponential factor of 10(6.1+/-2.0) (mol/cm(3))(0.34) s(-1). At higher water densities, hydrolysis becomes more important and the methanol yield increases with water density. Potential mechanisms for methylamine hydrolysis include S(N)2 and molecular mechanisms. (C) 2004 Elsevier B.V. All rights reserved.