The adsorption performance towards methylene blue (MB) from aqueous solution on the anaerobic granular sludge (AGS) was described. The adsorption isotherm data were analyzed by Scatchard plot, which showed that multiple classes of binding sites were involved in the present adsorption process. Three kinds of functional groups, including carboxyl, amino and phosphate group, were identified by potentiometric titration experiment. And the amount of these three functional groups was 0.12, 0.14 and 0.23 mmol/g, respectively. To determine their contribution in the adsorption process, anaerobic granules were used in adsorption after blocking specific functional group. Fourier transformed infrared spectroscopic (FTIR) study was carried out to characterize the modification of the functional groups due to chemical treatments. Carboxyl and amino groups were identified as the most important moieties involved in the binding process since blocking of these two functional groups caused a decrease in the adsorption capacity by 32.68% and 41.89%, respectively. However, the effect of phosphate group on adsorption was slight with the amount of adsorption capacity remaining stable. (C) 2010 Elsevier B.V. All rights reserved.