Iron oxide (α–Fe2O3) and cobalt oxide (Co3O4) were produced via precipitation reactions carried out in a continuous hydrothermal apparatus. The resulting particles were nanometer-sized because of the high supersaturations generated when metal nitrate solutions are combined with sodium hydroxide or with hot, compressed water. The average particle size increased with the metal nitrate feed concentration and with residence time. A logarithmic relationship was obtained between the particle size and feed concentration and between particle size and residence time in the apparatus. The production of nanoparticles with narrow size distribution was shown to require low metal nitrate feed concentrations and short residence times. In the range of temperatures studied in this work, temperature apparently had no effect on the size except when cobalt nitrate was contacted with supercritical water in the absence of sodium hydroxide. In this case, large cobalt oxide particles were obtained when the temperature was above the critical temperature of water.