The relationship between a liquid and its vapour at equilibrium. At a given temperature and pressure, the composition of the vapour will be determined by the composition of the liquid. The relationship between the two is frequently expressed using K-factor values of the components involved. For binary components A and B showing ideal behaviour, the relative volatility,_AB, can be used where the mole fraction of the vapour of the more volatile component y_A is related to the liquid mole fraction x_A by:

There are various liquid activity models available including Margules, van Laar, Wilson, non-random two-liquid (NRTL), and universal quasi-chemical (UNIQUAC) models. Mixing rules are used for mixtures to combine pure component parameters.