Biosorption of Cd2+ and Pb2+ onto sphagnum peat moss was investigated. The mole numbers and acid dissociation constants of metal-binding sites (carboxyl and phenolic hydroxyl groups) were determined by potentiometric titration. The Cd2+ and Pb2+ adsorption data were analyzed using two different adsorption models. The models were based on the acid dissociation reactions of acidic sites and the monodentate or bidentate binding reactions of bivalent metal ions to acidic sites. The results showed that the biosorption of bivalent metal ions onto peat moss followed the bidentate adsorption model. The number of acidic sites of peat moss was about 1.5 mmol . g(-1), so that the saturated adsorption amount of bivalent metal ions was 0.75 mmol . g(-1). It was also shown that the adsorption process of Cd2+ and Pb2+ followed pseudo-second-order kinetics.