The effect of somes parameters on the biosorption of cadmium has been studied using olive stones as the solid sorbent. Results show that the smaller the size of the sorbent particles, the higher the percentage of cadmium retrieved, up to 90% for sizes ranging between 0.355 and 0.250 mm. Biosorption was determined to be enhancing at pH approaching neutral, decreasing considerably when the acidity in the medium increased. The biosorption process was found to occur rapidly, i.e. the maximum sorption capacity was reached within 20 min. The process involves pseudo-second-order kinetics with an activation energy value within the normal range considered for processes, where a physical interaction between the sorbate and the sorbent solid predominates. (c) 2005 Elsevier Ltd. All rights reserved.