Four thermodynamic consistency tests are discussed along with sufficient background information so that approximations to these otherwise rigorous tests are fully appreciated. The tests are applied to three binary systems, one at low pressures, one at moderate pressures and the last at high pressures. The results provide information on the data and the models used to fit them. These results are highly sensitive to the selection of the models, a conclusion worth noting for those selecting physical property models in process simulators and performing PTx experiments. For example, the choice of an inappropriate equation of state for the representation of vapor-phase properties will introduce thermodynamic inconsistencies that cannot be removed by simply increasing the number of adjustable parameters in the activity coefficient expressions.