Abstract: Polyether-based thickeners for aqueous systems, said polyethers having molecular weights of about 1000 to about 25,000 which are modified by reaction with alpha-olefin oxides having about 12 to about 18 carbon atoms, exhibit increased thickening efficiency in the presence of certain organic additives. These are ethoxylated phosphate esters or mixtures of said esters with water-soluble amines. The polyethers contain about 1 to about 20 percent by weight, based upon the weight of the liquid polyether, of an alpha-olefin oxide. Polyether-based thickeners containing an ethoxylated phosphate ester, or said ester in admixture with a water-soluble amine, and thickened aqueous compositions are disclosed together with processes for the preparation of thickened aqueous systems. The polyether-based thickeners of the invention are particularly useful for thickening water or water-glycol mixtures which are useful as hydraulic fluids.