The synthesis of CeO2/ZrO2 core-shell nanocrystals (NCs), which are CeO2 NCs overcoated by a thin ZrO2 layer, has been studied together with CeO2 and ZrO2 NCs using a hot surfactant colloidal route. Colloidal solutions in oleylamine of the CeO2 NCs with a 2.0-2.5 nm diameter and ZrO2 NCs with a 2.5-3.5 nm diameter were, respectively, synthesized by the thermolysis of Ce(acac)(3) and the reaction of Zr[OCH(CH3)(2)](4) and ZrCl4. The CeO2/ZrO2 core-shell NCs have been successfully synthesized by the addition of CeO2 NCs as seed crystals in the ZrO2 source solution and its reaction at 300 degrees C. The core-shell nanostructure, which consists of a 2.4 nm diameter CeO2 core and 1.2 nm thick ZrO2 shell, was supported by its X-ray diffraction, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy results. (c) 2006 The Electrochemical Society.