In the application of supercritical water oxidation (SCWO) to the treatment of aqueous solid wastes, particle size will be an important processing parameter. The particle size will impact feed preparation requirements such as slurry concentration, pumping requirements, and, in terms of the destruction of the solid particles, rate of reaction and reactor size. To address these issues, an experimental research program was undertaken to evaluate the effect of particle size on the reaction kinetics in SCWO of solid particulates (wheat straw and cellulose particles in this case). The experiments also included evaluation of the effects of temperature, pressure, and agitation rate. A two-step reaction mechanism was revealed, with a very rapid initial dissolution period followed by a longer particle reaction period. Empirically based mathematical relationships were developed that can be used for SCWO system design.