Abstract: The real state of vapor liquid equilibria is considered as a combination of three fictitious states. From the relation between the real state and three fictitious states, the approximatep, y, x relationship can be obtained from these fictitious states with the help of the Gibbs-Duhem equation. This method was used to calculate isothermal high (low) pressure vapor liquid equilibria with satisfactory results, while no equations of state or equations of activity coefficients were needed.