Adsorption equilibria of 1-naphthol and 1-naphthylamine were investigated in single-solute and binary-solute (primary-co-solute and equimolar solute experiments) adsorption systems at 293 K. Two commercial nonpolar polystyrene adsorbents (NDA-16 and NDA-1800) were employed here for their frequent use in organic pollutants removal from contaminated waters. All the adsorption isotherms of 1-naphthol and 1-naphthylamine in single-solute and equimolar binary-solute adsorption systems were found to be well fitted by Langmuir equation. Adsorption uptake of the primary solute was enhanced in the presence of the co-solute, arising presumably from the cooperative effect caused by the laterally attractive acid-base interaction between the adsorbed 1-naphthol and 1-naphthylamine molecules. Due to the stronger adsorption affinity of 1-naphthylamine to nonpolar adsorbents, adsorption enhancement of 1-naphthylamine in the presence of 1-naphthol is greater than that of 1-naphthol in the presence of 1-naphthylamine. A model (modified extended Langmuir model, MLM) is proved to well describe the cooperative adsorption of 1-naphthol and 1-naphthylamine in the equimolar binary-solute system. The cooperative coefficient of one adsorbate is linearly correlated with the amount of the other adsorbed on the adsorbent. (c) 2006 Elsevier B.V. All rights reserved.