Abstract: An aqueous hydraulic fluid composition comprising (i) a salt of formic acid, and (ii) one or more lubricants such as a monovalent metal salt, ammonium, or amine salt of a dicarboxylic acid, is described in which the aqueous hydraulic fluid composition demonstrates increased thermal stability when exposed to elevated temperatures for a prolonged period of time while being able to tolerate the presence of 10% v/v synthetic seawater. The aqueous hydraulic fluid composition contains less than about 20% by weight (preferably none or substantially none) of an oil selected from the group consisting of mineral oils, synthetic hydrocarbon oils, and mixtures thereof. The hydraulic fluid preferably contains no glycols in some embodiments. The pH of the hydraulic fluid is preferably from 8 to 10 and is maintained by a buffer which preferably comprises borax in some embodiments.