Thermal properties were investigated for binary mixtures of water and five ionic liquids: 1-butyl-3-methylimidazolium iodide abmim][I]), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF(4)]), 1-butyl-3-methylimidazolium thiocyanate ([bmim][SCN]), 1-ethyl-3-methylimidazolium ethylsulfate ([einim][EtSO(4)]), and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([emim][TfO]). The thermal stability was examined by thermogravimetry. The phase behavior was investigated by differential scanning calorimetry from room temperature to 153 K, and phase diagrams were constructed. Vapor pressures were measured using a static method in the temperature range of (313 to 343) K and in the concentration range of mole fraction (0 to 0.65) of ionic liquids. It was found that [emim][TfO] + H(2)O mixtures are promising for working fluids in heat pipes. Loop heat pipe operation tests were also performed at (313 to 393) K using the mixtures.