Vapour pressure curves for simple fluids have been obtained using different equations of state, and predictions are compared with experimental data of argon. For this, a previous quantitative test of these equations should be made in order to determine their theoretical foundations, mathematical simplicity and accuracy in predicting the thermodynamic behaviour of these systems. The results show that, despite the simplicity of our models, the thermodynamics obtained from these models is in accordance with experimental data of argon on a wide range of temperatures and pressures. An extra advantage of our models is their theoretical background. It will permit a straightforward extrapolation to study other non-simple systems as polar fluids.