The nearly ideal binary solvent (NIBS) approximation is applied to gas-liquid partition coefficients derived from gas-liquid Chromatographic experiments with a mixed liquid phase. For the noncomplexing systems studied to date, the resulting equation can be reduced to the empirical equation in which partition coefficients in binary systems are linear with volume fraction. Extension of this model to a system with a 1:1 solvent-solute complex results in an equation which can also be reduced to the linear form in some circumstances. The linear relationship between volume fraction and the partition coefficient of a solute in a binary solvent is demonstrated to be a nongeneral thermodynamic relationship. © 1979 American Chemical Society.