A 25.4-mm diameter hydrocyclone with an underflow receiver was evaluated for its ability to achieve separation of fine particles from water at elevated temperatures and pressures relevant to supercritical oxidation. Temperature was varied from 25 degrees C to 340 degrees C, while pressure was maintained at 27.6 MPa. The particles studied were alpha-alumina. Particle-removal efficiency was affected by the separation capabilities of the hydrocyclone, deposition on the heater surface, and flocculation of the particles. Particle-size distributions and suspended solids analyses confirmed that cyclone, separation efficiency was controlled by the (density(particle) - density(water))/viscosity(water) ratio. Because this ratio is sensitive to temperature, especially in the neighborhood of the supercritical point, separation efficiencies sharply increased with temperature. Contrary to traditional air cyclone theory, removal efficiency was inversely correlated to flow rate. This result was caused by particle deposition and particle flocculation in the heater. Low now rates increased heater detention times and, thus, opportunities for flocculation and particle deposition. Therefore, the performance of a hydrocyclone used in conjunction with supercritical oxidation depends on phenomena occurring in the heater and the hydrocyclone.