The removal of thallium cation from aqueous solutions has been studied using pistachio hull as an adsorbent. The effect of variables such as solution pH, contact time and adsorbent amount has been investigated and these are optimized using a factorial design method. A model is developed to relate the content of adsorption to the variables. This model allows the prediction of the extent of adsorption at different conditions. Response surfaces are used for showing the effects of different variables on the content of adsorption. The analysis of variance is used to evaluate the validity of the model. There is no lack of fit found at the confidence level of 95%. The correlation coefficient between the calculated and the experimental data indicates good performance of the model. Under optimum conditions, a maximum removal of 96.5% is achieved. Both the Langmuir and Freundlich adsorption isotherms are found to be suitable for describing the biosorption of thallium ions onto pistachio hull.