The removal of zinc ions from aqueous solutions on the biomass of Azadirachta indica bark has been studied by using batch adsorption technique. The biosorption studies were determined as a function of contact time, pH, initial metal ion concentration, average biosorbent size and biosorbent dosage. The equilibrium metal uptake was increased and percentage biosorption was decreased with an increase in the initial concentration and particle size of biosorbent. The maximum zinc biosorption occurred at pH 6 and percentage biosorption increases with increase in the biosorbent dosage. Experimental data obtained were tested with the adsorption models like Langmuir, Freundlich and Redlich-Peterson isotherms. Biosorption isothermal data were well interpreted by Langmuir model with maximum biosorption capacity of 33.49 mg/g of zinc ions on A. indica bark biomass and kinetic data were properly fitted with the pseudo-second-order kinetic model. (c) 2007 Elsevier B.V. All rights reserved.