Binary liquid-liquid equilibria (LLE) for mixtures of dihydroxy alcohols and three imidazolium-based ionic liquids (ILs) were measured. The dihydroxy alcohols were 1,3-propanediol, 1,4-butanediol, and 1,5-pentanediol, and the ILs were 1-ethyl-3-methyimidazolium tetrafluoroborate ([C(2)C(1)im][BF4]), 1-ethyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide ([C(2)C(1)im][NTf2]), and 1-ethyl-3-methylimidazolium 1,1,2,2-tetrafluoroethanesulfonate ([C(2)C(1)im][TFES]). The experimental LLE data were well correlated using the nonrandom two-liquid (NRTL) activity coefficient model. All binary diol systems with [C(2)C(1)im][BF4] or [C(2)C(1)im][NTf2] have an upper critical solution temperature (UCST) between 310 and 360 K. An equimolar mixture of diols and [C(2)C(1)im][TFES] showed complete miscibility between 293.15 and 373.15 K. An increase in the alkyl chain length of the dihydroxy alcohols and/or changing the anion from [BF4-] to [NTf2 ] for a given [C(2)C(1)im] cation results in an increase in the UCST. The excess molar volume of diols with ILs was in most cases larger than those of ordinary solutions.