Supercritical water provides an excellent reaction medium for hydrothermal synthesis of metal oxide nanoparticles. In this work, zinc oxide nanoparticles are produced by oxidation of zinc acetate in supercritical water in a continuous tubular reactor. Less than I min is enough for the reaction to occur, and the temperatures are much lower than in the flame combustion or zinc evaporation processes. Effect of flow rates and feed concentrations are studied. Based on transmission electron microscopy and dynamic light scattering analyses, spherical zinc oxide particles are obtained in the range 39-320 nm. (C) 2002 Elsevier Science B.V. All rights reserved.