Surface-modified cerium oxide (CeO2) nanoparticles, dispersible in either a hydrophilic or a hydrophobic medium, are synthesized continuously in supercritical methanol using methoxy polyethylene glycol (PEG 350, H(OCH2CH2)(n)OCH3, MW = 350 g/mol) or alpha,omega-Bis(2-carboxymethyl)polyethylene glycol (PEG600, HO OCCH2(OCH2CH2)(n)OCH2COOH, MW = 600 g/mol) as a surface modifier. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show that the surface modifiers inhibit the growth of the particles, resulting in smaller size particles (20-25 nm) as compared to unmodified particles (35 nm) synthesized in supercritical methanol or unmodified particles (91 nm) synthesized in supercritical water. At a high concentration of PEG600 (0.3 M), surface-modified particles with a size of 3-4 nm and a low degree of aggregation are produced. An X-ray diffraction analysis reveals that the PEG-modified nanoparticles retain the CeO2 phase. Fourier transform infrared spectroscopy and a thermal gravimetric analysis indicate that the amount of modifier attached to the surface of the nanoparticles is 6.88%, when 0.3 M of PEG350 is used, and 4.49%, when 0.3 M of PEG600 is used. A long-term stability test (40 days) revealed that the PEG350-modified CeO2 nanoparticles have good dispersibility in a hydrophobic medium (oil), while PEG600-modified CeO2 nanoparticles have good dispersibility in a hydrophilic medium (water). This indicates that the nanoparticle dispersibility in either a hydrophilic or hydrophobic medium can be controlled by adjusting the end-group functionality and chain length of the modifiers. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.