Hydrofluorocarbon (HFC) + ionic liquid (IL) systems can be adopted as working pairs for compression/absorption hybrid cycles. A method of selecting novel working pairs has been proposed in this paper, that the infinite dilution activity coefficients (gamma(infinity)(1)) of working pair system need to be obtained, and the UNIFAC model is implemented to predict the activity coefficients of the working pair system. With the help of published experimental vapor-liquid equilibrium (VLE) data of 18 HFC + IL systems, 16 new group interaction parameters for four anion ILs and nine HFCs have been fitted to extend the evaluating and predicting range of HFC + IL systems for developing alternative working pairs. To validate the reliability of the method, the model parameters have been used to calculate the VLE data with the average relative deviations (ARDs) of pressures less than 8.5%. The prediction of the gamma(infinity)(1) shows that the solubility and affinity between HFCs and ILs increase with the decrease of the gamma(infinity)(1) which is in agreement with the experimental results. It is found by the prediction that the combinations of R32/R134 and [Tf2N]-based Its may be two kinds of potential working pairs to improve the performances of compression/absorption hybrid cycles.