We have studied the use of colloidal zirconia as a starting material for organic modification by covalent grafting. The zirconia particles have been obtained by thermolysis of zirconium acetate. They are built of 60 nm aggregates of smaller primary particles (5 nm) of well crystallised monoclinic zirconia with a surface covered by acetate groups. Reaction with phosphoric acid does not affect the inner structure of the monoclinic zirconia but leads to a complete replacement of acetate groups by acid phosphates. A maximum of 5.0 x 10(-5) S cm(-1) is obtained for the pressed powders (20 degreesC, 90% relative humidity). A higher conductivity (1.3 x 10(-3) S cm(-1) at 90% R.H.) can be obtained by the grafting of metasulphophenylphosphonic acid (SPPA), although the particle coverage is limited by steric hindrance. Finally, direct reaction of SPPA with zirconium propoxide yields highly dispersed colloidal particles (5 nm), with a large concentration of sulphonate and hydroxyl groups. As a consequence, the dried materials exhibit a high proton conductivity, stable at moderate relative humidity (>2 x 10(-3) S cm(-1) at R.H. as low as 50%). (C) 2003 Elsevier B.V. All rights reserved.