Adsorption equilibrium constants (K) of three aromatic acids on CM-Sephadex C-50 and DEAE-Sephadex A-25 columns were determined by using isocratic elution chromatography and the first absolute moment of elution curves. The column efficiency was evaluated by the height equivalent to theoretical plate (HETP) in plate theory, and the values of HETP were calculated from the first absolute moment and the second central moment. The effects of pH and ionic strength on the values of K and HETP were investigated.