Adsorption of thallium(M) ions from aqueous solutions onto eucalyptus leaves powders as a low cost adsorbent has been studied in batch mode. It is observed that modification of the adsorbent with basic solution (NaOH solution) is essential for achieving a significant adsorption. The alkaline modified adsorbent shows a significant amount of adsorption. However, the adsorbent treated with nitric acid shows no activity. The effects of experimental conditions such as initial solution pH, agitation speed, contact time, adsorbent amounts, initial thallium concentration and temperature are studied. The results show that adsorption is strongly pH-dependent. The maximum adsorption of thallium(III) ion obtained at 25 +/- 1 degrees C is found to be 78.2%. The equilibrium data are described by the Langmuir isotherm but a slightly inferior fit is obtained using the Freundlich model. The monolayer saturation capacity is calculated to be 217.4 mg g(-1). The separation factor indicates that eucalyptus leaves powders are favorable for the sorption of thallium(III).