Abstract: An aqueous solution comprising a thickening polymer with diol groups distributed along it, such as guar or other polysaccharide, is cross linked with a cross-linker which contains a plurality of boroxole groups of the partial formula wherein R1 is hydrogen or a substituent group or an attachment to the remainder of the cross-linker molecule, R2 is hydrogen or a substituent group, R3 is hydrogen or an aliphatic or aromatic group and the carbon atoms joined by a double bond are part of an aromatic ring. The thickened fluid may be a wellbore fluid and may be a hydraulic fracturing fluid in which a particulate proppant is suspended.