The adsorption behavior of pentachlorophenol (PCP) from aqueous solution to activated sludge biomass was quantitatively characterized in this paper. The effects of the initial pH value and biomass concentration on bioadsorption were investigated. The Freundlich adsorption isotherm was applied to describe the biosorption processes and the isotherm constants were evaluated. The experimental results indicated that the initial pH value and biomass concentration are important parameters affecting the adsorption capacity that increased with decreasing biomass concentration (in the range less than 5 g/l) and pH (between 6 and 8). Both the biomass concentration and pH value only affected the capacity constant K-F of the Freundlich equation while the intensity constant n remained constant.