A simple model for obtaining important information on the retention and the resolution of proteins by electrostatic interaction chromatography is described. The relationships among different definitions of distribution coefficients are explained in detail. The method employs a linear gradient elution experiment, in which a salt concentration is increased linearly with time at a fixed pH. The experimental data for electrostatic interaction chromatography and hydroxyapatite chromatography of proteins have shown that the number of binding sites values (B) determined from the experimental data as a function of pH can provide instructive information oil the retention and the optimization of protein separations.