A vapour compression simulation model was developed. Simple mathematical models were employed for each component of the cycle. They resulted in a set of nonlinear equations, which was solved numerically. Heat losses from condenser to ambient were included. The model is capable of predicting the operating point of the system (including condensing and evaporating pressures) as a function of equipment characteristics (for example, compressor swept volume, speed and clearance ratio, and heat exchanger overall conductances) and prevailing thermodynamic conditions (such as heat source and heat sink temperatures with the mass flow rates of their fluids). The predicted performance was compared to that of an existing R-12 unit, showing good agreement. As an application, a comparative analysis is made on the thermodynamic performance of a domestic heat pump running on two different refrigerants: R-12 and R-134a.