An extended Radke-Prausnitz equation is useful in predicting the adsorption equilibria of organic multi-solute aqueous solutions on activated carbon. It has an advantage that, if the single-solute and two-solute parameters are available, the calculation of adsorption equilibria for more than three-solute systems can be carried out without additional parameters. The authors have previously proposed an empirical method for estimating the single-solute parameters using the physical properties of solutes and have obtained good prediction results for thirteen single-solute systems. In this study, the two-solute parameters of the extended Radke-Prausnitz equation are correlated with the normal boiling point and molecular weight of solutes and the prediction performances are examined for adsorption equilibria of six two-solute aqueous solution systems. The prediction results are in good agreement with the experimental data.