If the liquid and vapor are pure, in that they consist of only one molecular component and no impurities, then the equilibrium state between the two phases is described by the following equations:
:; and
where  and  are the pressures within the liquid and vapor,  and  are the temperatures within the liquid and vapor, and  and  are the molar Gibbs free energies (units of energy per amount of substance) within the liquid and vapor, respectively. In other words, the temperature, pressure and molar Gibbs free energy are the same between the two phases when they are at equilibrium.
An equivalent, more common way to express the vapor–liquid equilibrium condition in a pure system is by using the concept of fugacity. Under this view, equilibrium is described by the following equation:
where and are the fugacities of the liquid and vapor, respectively, at the system temperature  and pressure . It is often convenient to use the quantity , the dimensionless fugacity coefficient, which is 1 for an ideal gas.