The comparative study on adsorptions of Pb(Ⅱ) and Cr(Ⅵ) ions by free cells and immobilized cells of Synechococcus sp.was performed,in which different aspects including Zeta potential of the cells,the influence of pH,temperature and initial concentration of metal ions,as well as adsorption kinetics and mechanism were referred.The lyophilized free cells have a surface isoelectric point at pH 3,and the correlative experiment indicates that there is an electrostatic adsorption feature of Cr(Ⅵ) and Pb(Ⅱ).The immobilization of the free cells by Ca-alginate does not significantly modify the adsorption features of the biosorbent.The absorption processes of Cr(Ⅵ) and Pb(Ⅱ) on both free and immobilized cells are apparently affected by pH and the initial concentration of metal ions in the bulk solution,but are much weakly affected by temperature in the test range of 10-50 ℃.The slow course of biosorption follows the first order kinetic model,the adsorption of Pb(Ⅱ) obeys both Langmuir and Freundlich isotherm models,while the adsorption of Cr(Ⅵ) obeys only Freundlich model.FT-IR results indicate that carboxylic,alcoholic,amide and amino groups are responsible for the binding of the metal ions,and reduction of Cr(Ⅵ) to Cr(Ⅲ) takes place after Cr(Ⅵ) adsorbs electrostatically onto the surface of the biosorbents.