Isothermal compressibilities of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-ethyl-3-methylimidazolium tetraisothiocyanato-cobaltate(II) (I) 1-butyl-3-methylimidazolium tetraisothiocyanato-cobaltate(II) (II), and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide were compared with ref. mineral oil and a biodegradable oil. Temp. and pressure relationships of isothermal compressibility of liqs. under test were discussed for their use as hydraulic fluids. The I and II salts showed the lowest compressibility.