An introduction is given to different thermodynamic models for representing mixing enthalpy measurements. The TAP series and the associate model are applied and compared on the basis of the concentration and temperature-dependent mixing enthalpy of the system Mg-Pb. A new appoximation method is introduced to calculate the parameters of the associate model. The number of available enthalpy values for the approximation process is systematically varied in order to simulate the flexibility of the models. From this point of view the associate model is a useful model for the representation of the mixing enthalpy and the prediction of the excess heat capacity.