In a previous paper, a mathematical model was developed for the estimation of binding constants by capillary electrophoresis (Guillaume and, Peyrin, Anal. Chem. 1999, 71, 2046-2052). This model was applied to determinate the binding constant of the guest-vancomycin complex as well as the degree of complexation n(c) (percent of complexed guest), In this work, the guest molecule were a series of acid herbicide enantiomers (aryloxypropionic, aryloxyphenoxypropionic, and aminopropionic acid). The effects of the run buffer salt concentration on the herbicide-vancomycin binding was clearly described by the determination of specific physicochemical parameters as well as thermodynamic data. These results demonstrate that a better understanding of the interactions involved, in the guest molecule-vancomycin binding allows to optimize the enantiomeric separation of this chiral guest molecule.