A new procedure has been developed to investigate the capability of an equation of state to predict in a mathematical way an experimental inversion curve. Application of the proposed method to the four cubic equations of state van der Waals, Redlich-Kwong, Soave and Peng-Robinson in their ability to predict the experimental inversion curve of nitrogen has indicated that the Redlich-Kwong equation of state gives the best results. The proposed procedure was also used to find the best of three well known mixing rules, using the Redlich-Kwong equation of state and the experimental inversion curve of air.