Organic modification of nano-SiO2 particles was performed with supercritical CO2 as solvent and a titanate coupling reagent NDZ-201 as the modification reagent. The surface of the nano-SiO2 particles was changed after modification by the titanate coupling reagent from hydrophilic to hydrophobic. IR and thermo-gravimetric analyses indicate that the interaction between the titanate coupling reagent and the particle surface is mainly through chemical bonding. The influence of the apparent concentration of the titanate coupling reagent on the modification at 60 degrees C and 20.0 MPa was investigated, in which the apparent concentration is the weight percent of the titanate coupling reagent added in the supercritical solvent. The quantity of the titanate coupling reagent reacted on the particle surface reaches a maximum of 19.81 X 10(-7) mol/m(2) i.e. 1.19 molecule titanate coupling reagent/nm(2), when the apparent concentration of titanate coupling reagent is 0.6% (wt). (C) 2005 Elsevier B.V. All rights reserved.