Biosorption of Hg (II) was investigated in this study by using guava bark powder (GBP). In the batch system, effects of various parameters like contact time, initial concentration, pH and temperature were investigated. Removal of Hg (II) was pH dependent and was found maximum at pH 9.0. Based on this study, the thermodynamic parameters like change in standard Gibb's free energy (Delta G(0)), standard enthalpy (Delta H-0) and standard entropy (Delta S-0) were evaluated. The rate kinetic study was found to follow second-order. The applicability of Freundlich adsorption isotherm model was tested. The value of regression coefficient was greater than 0.99. This indicated that the isotherm model adequately described the experimental data of the biosorption of Hg (II). Maximum adsorption of 3.364 mg g(-1) was reached at 80 min. The results of the study showed that guava bark powder can be efficiently used as a low-cost alternative for the removal of divalent mercury from aqueous solutions. (C) 2008 Elsevier B.V. All rights reserved.