In liquid-phase adsorption of benzene + heptane on NaHY zeolites containing sodium, surface excesses were measured at 15, 30, and 45 degrees C. Maximum surface excesses appeared within the range of benzene mole fractions between 0 and 0.2 and increased with the sodium content. The total adsorption amount, separation factor, and difference between heats of immersion in two pure adsorbates were determined by using Everett’s adsorption model. These thermodynamic parameters increased with the sodium content, especially rapidly above 27 sodium ions per unit cell.