The adsorption of ampicillin onto natural and organo-bentonite was studied. Organo-bentonite was obtained by modifying the natural bentonite obtained from Ponorogo. Indonesia, using CTAB surfactant by microwave heating. The temperature dependent form of the Langmuir, Freundlich, Sips and Toth equations was employed to correlate equilibrium data. Based on the evaluation of the physical meaning of fitted isotherm parameters of each model, it is clear that Toth equation can represent the equilibrium data better than other models. The adsorption performance of natural and organo-bentonite for the removal of ampicillin from pharmaceutical company wastewater was also studied. In real wastewater, both adsorbents could not completely remove the ampicillin due to the sorption competition with other substances which also present in the wastewater. (C) 2011 Elsevier B.V. All rights reserved.