Colloidal monoclinic zirconia ZrO2 particles have been synthesized by hydrothermal treatment from acetate solutions. To increase their surface acidity, they have been treated by aqueous solutions of phosphoric acid, sulfophenylphosphonic acid (SPPA, (HO)(2)(O)-C6H6-SO3H) and sulfodifluoromethylphosphonic acid (SFPA, (HO)(2)(O)P-CF2-SO3H). This leads to the covalent bonding of phosphoric or sulfortic acid groups onto the surface of the particles. Solid state NMR (P-31, H-1) studies show the covalent grafting of the phosphate and phosphonates groups and qualitatively illustrate the fast proton dynamics of these surface conducting materials as compared with that of crystalline alpha-Zr(HPO4)(2). H2O. But, water adsorption is still necessary to increase the long distance proton mobility. Then, the macroscopic conductivity remains low (between 10(-4) S cm(-1) and 10(-3) S cm(-1) 25 degrees C, RH 70%) and shows a strong hysteresis while cycling the relative humidity. The mechanism limiting the conductivity seems to be interparticle transfer. (c) 2006 Elsevier B.V. All rights reserved.