Abstract Adsorption of fumonisin B 1 (FB 1) on natural clinoptilolite-rich zeolitic tuff, modified with different amounts (2, 5 and 10 mmol M +/100 g) of octadecyldimethylbenzyl ammonium (ODMBA) ions, at pH 3, 7 and 9, was investigated. The organozeolites were denoted as OZ-2, OZ-5 and OZ-10. Results showed that the presence of ODMBA ions at the zeolite surface greatly improved adsorption of FB 1, especially at pH 7 and 9. FB 1 adsorption on all three organozeolites showed a nonlinear type of isotherm, at pH 3 and 7 and 9. For OZ-2, the highest adsorption of FB 1 was achieved at pH 3, while for OZ-10, the highest FB 1 adsorption was observed at pH 9. At pH 7 and 9, FB 1 adsorption was lowest for OZ-2, and increased as the amount of ODMBA at the surface increased. The highest FB 1 adsorption was achieved when the zeolite surface was totally covered with ODMBA. Since OZ-2 has 20% of inorganic cations exchanged with ODMBA, the lower FB 1 adsorption at pH 7 and 9 may be the result of the presence of the anionic form of FB 1 that can not be adsorbed at the uncovered negatively-charged surface of the modified zeolitic tuff. The estimated maximum FB 1 adsorption to OZ-10, based on a Langmuir fit to the data, was greatest (10.819 mg/g) at pH 3, suggesting that cationic form of FB 1 also contributes to adsorption. Based on the structure of FB 1 and the obtained adsorption data, it is possible that more than one sorption mechanism is relevant for FB 1 adsorption by ODMBA modified zeolites.