We here report on the covalent grafting of various phosphated species (phosphoric acid, phenylphosphonic acid, and octyl phosphate) onto the surface of monoclinic zirconia nanoparticles obtained by hydrothermal treatment of zirconium acetate. The initial particles are 60 nm aggregates of nanometric primary grains and present an inner porosity. Small-angle X-ray scattering shows that the high specific area of the colloidal particles (450 m(2.)g(-1)) decreases to 150 m(2.)g(-1) upon drying. Therefore, phosphated reactants can access the whole internal surface of the aggregates only before drying. The surface of the particles can be covered with functional groups bound through a variable number of Zr-O-P bonds. Several factors probably enhance the reaction between the particles and the phosphates or phosphonates: the large specific area of the particles, a fully accessible porous network, and a large concentration of surface terminal groups. At the same time, the morphology of the particles is well preserved upon grafting. This is due to the good crystallinity of the primary grains that constitute the particles. In addition, the grafting drastically modifies the surface properties of the colloids. For example, the polarizability of the particles decreases in the sequence -POH > as-prepared ZrO2 > -PC6H5 > -POC8H17. Furthermore, the grafting of octyl phosphate allows exclusion of water from pores of 2 nm radius, up to hydrostatic pressures of 20 MPa.