Two equations of (r/x)x=0-a modified Kikic-Renon equation and a Valentin-Guiochon equation-have been studied. Values of r1∞ and (dlnr1/dx1)x1=1 by these two equations for 29 binary systems

have been calculated from GLC data

the parameters in some models of excess Gibbs free energy evaluated

and the vapor-liquid equilibrium date predicted. The results are satisfactory. Vlentin-Guiochon equation was simplified leading to a simpler (r/ x)x=0 expression. Then values of r1∞ and (dlnr1/dx1)x=0 by the simplified Valentin-Guiochon equation for 56 binary systems containing alcohols or water were taken from Gmehlings VLE data collection

and the same predicting procedures repeated. These procedures have also been applied to 43 binary systems containing various polar and nonpolar

volatile and nonvolatile component

in which the values of r1∞ and) dlnr1/dx1)x1=0 were calculated by the simplified Valentin-Guiochen equation from GLC data. The predicted VLE data are all consistent with those in the literature. Limits of application of the three equations and methods to transform the two-parameter model of excess Gibbs free energy into one-parameter model

were then discussed.