In response to the phasing out of CFCs in refrigeration applications, tetrafluoroethane (R134a) refrigerant and polyol ester lubricants have been developed to replace the traditional R12/mineral oil systems. This article describes a statistical thermodynamic method that has been used to model the thermodynamic behaviour of R134a and an ISO 15 polyol ester (i.e. a polyol ester having a viscosity of 15cSt at 40C). The molecular interaction and the excess chemical potential of the mixtures is calculated by simulating the vapour pressure of the mixture and comparing it with some available experimental data. The phase diagram of the mixture is successfully simulated using a modified equation-of-state thermodynamic model. The result of the theoretical simulation is compared with experimental findings. Excellent agreement is obtained. Other quantities such as change in free energy, enthalpy, entropy and volume change on mixing are calculated for the temperature range -40 degrees C to +100 degrees C.