We first report application of ionic liquids to working fluids used in an absorption chiller. [bmim][BF4]+TFE and [bmim][Br]+TFE mixtures were chosen as a potential working fluids to overcome serious disadvantages of the conventional working fluids. Vapor pressures were measured by using the boiling point method in the concentration range of 40 to 90 ionic liquid mass percent and correlated with an Antoine equation. Heat capacity measurements of each pure ionic liquid were carried out in the temperature range of 293.15 to 323.15K and correlated with an simple polynomial equation. Resonable agreements were obtained between calculated and experimental values. Br anion was found to be better than BF4 in the aspect of the affinity with TFE