Investigation of derived thermodynamic properties using Sandler-Wong mixing rules shows that predicted excess properties can be unreliable since they are a complex function of the original Gibbs free energy model and the a and b terms. Sometimes the mixing rule will predict a different qualitative behaviour for the heat of mixing when compared to the one predicted by the original Gibbs excess model. Moreover the mixing rules can produce thermodynamic inconsistency at elevated pressures and lead to the prediction of negative heat capacities.