THE advantages of using binary data to calculate the effect of a high-boiling solvent on the volatility of a pair of liquids have frequently been emphasized. Recently, a thermodynamic method of calculating the average value of the logarithm of the volatility ratio of the low-boiling pair (components 1 and 2) at a constant mole fraction of the high-boiling solvent (component 3) under isothermal conditions from binary data has been advanced1. A treatment of this type has now been developed for single liquid-phase systems under a pressure of one atmosphere and the following approximate expression obtained These terms have the following significance: is the average value of the logarithm of the volatility ratio of component 1 with respect to component 2 at a constant mole fraction, x3′ of the high-boiling solvent taken from the composition x1→0 to x2→0 under isobaric conditions. Thus this quantity is defined by equation (2), where the integral on the right-hand side is to be taken for a pressure of one atmosphere and for a constant mole fraction, x3′, of component 3. ΔT21 is the normal boiling point of pure component 2 minus the normal boiling point of pure component 1, in degrees Celsius; ΔT23 is the normal boiling point of the mixture of components 2 and 3 containing a mole fraction x3′ of the extractive solvent 3, minus the normal boiling point of pure component 2, in degrees Celsius; ΔT13 is the normal boiling point of the mixture of components 1 and 3 containing a mole fraction x3′ of the extractive solvent 3, minus the normal boiling point of pure component 1, in degrees Celsius; T is the mean of the normal boiling points in degrees absolute of the mixtures (1,3) and (2,3) containing a mole fraction x3′ of component 3.