Abstract Isotherms of adsorption of Azraq humic acid (HA) onto Azraq bentonite were studied at different pH (3.0–6.0) and temperature values. The Langmuir monolayer adsorption capacity ( q m) was found to be 53 mg HA /g raw bentonite at pH = 3.0 and 25 °C, Δ H° = 54.8 kJ/mol, Δ S° = 220 J/mol deg which were approximate to those reported for the adsorption of HA on kaolinite. In the case of bentonite, the q m and Langmuir constant ( K L) values are significantly greater than those reported for zeolite. These results indicate the importance of sheet structure of bentonite in the adsorption process. Treatment of raw bentonite with NaCl, CH 3COOH and HCOOH does not affect adsorptivity ( q m and K L) of HA on bentonite, while treatment with HCl causes great decrease in adsorptivity. The adsorptivity of HA on bentonite was found to be inversely proportional to the increase in pH because of the increase in repulsion between HA and bentonite.