This study is a first step towards the prediction of the behaviour of working fluids and compressor oils. Different thermodynamic models are investigated and a modified Flory-Huggins-based predictive model, termed MFH, is proposed for solubility of HFCs in long-chained hydrocarbons. From experimental data, 7 general parameters have been fitted for the MFH model to describe 14 binary systems. The UNIQUAC model demands 28 specific parameters without the possibility of extending their validity to other mixtures. A predictive UNIFAC model requires 8 general parameters, but its uncertainty, 17%, is poor compared to the MFH model’s 7%.