Advanced space life support systems, especially systems that include growing plants to produce food, require the recovery of resources - primarily carbon dioxide and water - from various hydrocarbon wastes. Supercritical water oxidation (SCWO) of wastes is one of several possible techniques for oxidizing waste organics to recover the carbon dioxide and water. Supercritical water oxidation has the advantages of fast kinetics, complete oxidation, and the minimization of undesirable side products. However, the SCWO process requires further development before the process can be implemented in space life systems. One of the SCWO development needs is in the area of destruction of insoluble solids - such as inedible biomass or human wastes. Insoluble solids have to be introduced into a SCWO reactor as particles, and it can be expected that the particle size of the solids will affect the rate of reaction. Once the effects of particle size are understood, then the requirements for grinding or other preparation of feed solids before introduction into the reactor can be defined. To study the effects of particle size an experimental effort was begun at NASA Ames Research Center. Particles of polystyrene bead ranging in diameter from 3.4 μm to 220 μm were tested. A significant particle size effect on the rate of reaction was measured at 720°F.