Abstract: An aqueous fluid contains an aqueous solution or dispersion of a polymer to thicken the fluid together with a cross linking agent to enhance the viscosity of the fluid by crosslinking the polymer, wherein the crosslinking agent comprises supporting structures bearing functional groups to react with the polymer molecules and has a mean particle size of 2 nanometer or more. The supporting structures may be nanoparticles and the functional groups may be boronic acid groups. The concentration of boron in a thickened fluid may be low and in some instances there is resistance to applied pressure. The fluid may be a hydraulic fracturing fluid.