A B S T R A C TThe adsorption of Acid Blue 129 (AB129) from aqueous solution onto hydrochloric acid-activated montmorillonite clay (HCl-bentonite) was investigated. The activated clay was characterized by scanning electron microscopy, energy dispersive spectroscopy, and BrunauerEmmett-Teller surface area. Batch adsorption experiments were performed to investigate the effects of pH, contact time, initial dye concentration, and temperature (10, 20, 30, and 40˚C). Acidic conditions was suitable for higher adsorption of AB129, and kinetic studies demonstrate that the process followed a pseudo-second-order model. An activation energy of 23.858 kJ mol −1 was obtained for adsorption process. Adsorption data were fitted to Freundlich and Langmuir isotherms and various adsorption parameters have been calculated. Standard enthalpy (ΔH˚) and standard entropy (ΔS˚) were −44.90 kJ mol −1 and −68.44 kJ mol −1 K −1 , respectively, showing that overall adsorption process was exothermic and is spontaneous in nature with a decrease in the disorder of the system at the dye/adsorbate interface. However, the mechanism of the dye-Bentonite interaction is likely to be very complicated, involving a wide range of sites having different energy considerations. The activated clay was effective toward adsorption of AB129. The results show that activated bentonite clay could be employed as low-cost materials for the removal of acid dyes from colored effluents.