Single- and bisolute competitive adsorptions were carried out to investigate the adsorption behavior of an anionic dye, Orange II, and an anionic surfactant, dodecylbenzenesulfonate (DBS), from water at 25 degrees C onto montmorillonite treated with hexadecyltrimethylammonium (HDTMA) cation. The monovalent HDTMA cation was exchanged for the metal cations on the montmorillonite to prepare HDTMA-montmorillonite, changing its surface property from hydrophilic to organophilic. Adsorption affinity of DBS on HDTMA-montmorillonite was found to be slightly higher than that of Orange II, probably due to hydrophobic or nonpolar interactions between the long hydrocarbon chains of the HDTMA with an anion. The single-solute adsorption data were fitted to the Langmuir and the Redlich-Peterson (RP) adsorption models. The competitive Langmuir model and the ideal adsorbed solution theory (IAST) coupled with the single-solute adsorption models were used to predict the bisolute competitive adsorption equilibria. Among the models, the IAST coupled with the Langmuir and the RP models for DBS and Orange II, respectively, yielded the most favorable representation of the bisolute competitive adsorption behavior.