This paper reports on the research to develop a fire-resistant hydraulic fluid for aerospace vehicles. Model compounds containing halogen, phosphorus, and nitrogen in five chemical classes are characterized and compared to currently used materials with emphasis on rheological properties and fire resistance as measured by autogenous ignition, 1300 F molten metal test, flash and fire points. The chemical classes covered were: (1) partially fluorinated aliphatics (hydrocarbons, silicates, esters, amines), (2) halogenated ω,ω′-bis(aryldimethyl-silyl)alkanes, (3) fluoroaromatic ethers, (4) halogenated ω,ω′-diphenylalkanes, and (5) phosphorus esters.Presented as an American Society of Lubrication Engineers paper at the Lubrication Conference held in San Francisco, Calif., October 18–20, 1965.