Experiments for single and bisolute competitive adsorption were carried out to investigate the adsorptionbehavior ofβ-naphthalenesulfonic acid (NSA) and sulfuric acid from their solution at 25℃ onto weakly basic resinD301R. Adsorption affinity of sulfuric acid on D301R was found to be much higher than that of NSA. The dataof single-solute adsorption were fitted to the Langmuir model and the Freundlich adsorption model. The idealadsorbed solution theory (IAST) coupled with the single-solute adsorption models were used to predict the bisolutecompetitive adsorption equilibria. The IAST coupled with the Langmuir and the Freundlich model for sulfuric acidand NSA, respectively, yields the favorable representation of the bisolute competitive adsorption behavior.