A new Thermodynamic Approach for modelling mixtures of refrigerants and lubricating oils is presented. The model includes generalized methods for predicting the following thermodynamic properties of refrigerant-oil mixtures: bubble point temperatures, local oil concentrations, liquid specific heats release (enthalpy) curves are easily generated and also the effect of oil on the LMTD of evaporators can be modelled. A new definitionof the boiling heat transfer coefficient based on the bubble point temperature is also presented. The new Thermodynamic Approach will allow major advances to be made in two-phase refrigeration heat transfer research and design since now the type of oil and the effects of its physical properties can be included in the analysis.