Water-based hydraulic fluids and metalworking lubricants are disclosed which are thickened with a polyether polyol having a molecular weight of about 1000 to about 75,000 modified with an alpha-olefin epoxide having about 12 to about 18 carbon atoms. Unexpectedly, synergistic thickening results from a combination of said polyether polyol with the components of a water-based hydraulic fluid or metalworking lubricant. The particularly effective components of the hydraulic fluid or metalworking lubricant are the phosphate ester and water-soluble amine corrosion inhibitor components. The hydraulic fluid and metalworking fluids of the invention also contain a water-soluble polyoxyethylated ester of an aliphatic acid and a monohydric or polyhydric aliphatic alcohol, either one or both said acid and said alcohol being polyoxyethylated, a sulfurized molybdenum or antimony compound and a metal deactivator as well as other adjuvants conventional in this art.