A binding study on ␣ 1 -acid glycoprotein (AGP), a widely used chiral stationary phase in drug analysis, has been performed. Being a selectivity decisive factor in enantiomer separation, the adsorption of the organic modifiers, as the regularly used acetonitrile (AcCN) and also of dioxane, was determined from phosphate buffer eluents (pH 7.2 and 4.0) in the concentration range of 0.57-3.81 M and 0.11-1.9 M, respectively. The adsorbed amount was determined by gas chromatography. At lower modifier concentrations no significant difference was found in the binding extent of AcCN and dioxane. At higher, characteristic concentrations of the organic additives, a saturation was obtained at both pH values; furthermore, at pH 4.0 it was followed by a definite rise in the modifier adsorption. This particular behaviour may indicate the exposure of new binding sites on AGP surface, as a result of changes in the protein structure, which was confirmed by CD-spectroscopic measurements. The pH dependence of binding in equimolar concentration (1.9 M) has shown the priority of dioxane at pH 4.0, while the adsorption of AcCN dominated at higher pH, indicating the different character of the two solvents. The increased hydrogen bond formation should cause the preferred adsorption of dioxane at pH 4.0.