Silver was immobilized onto the surface of a chitosan membrane to examine the anti-biofouling performance of the membrane surface. The chitosan base membrane (denoted as CS) was first immobilized with ionic silver (denoted as CS_Ag(+)) and then the CS_Ag(+) membrane surface was chemically treated to obtain the membrane surface with reduced or metallic silver (denoted as CS_Ag(0)). The oxidation states of the immobilized silver on CS_Ag(+) and CS_Ag(0) and the interaction between silver and CS were investigated with X-ray photoelectron spectroscopy (XPS). The stability of the immobilized silver on the two types of membrane surfaces was evaluated through a leaching test. It was found that silver was effectively immobilized onto CS through surface complexation and the immobilized silver on CS_Ag(0) was at a reduced or lower oxidation state and was more stable than that on CS_Ag(+). Anti-bacterial and anti-biofouling experiments for CS. CS_Ag(+) and CS_Ag(0) were conducted with two types of typical bacteria, i.e., E. coli and Pseudomonas sp. From the disk diffusion tests (for 24h), it was found that, as compared to CS, both CS_Ag(+) and CS_Ag(0) showed significantly improved anti-bacterial performance, even though the CS_Ag(+) membrane surface seemed to exhibit slightly stronger anti-bacterial effect than the CS_Ag(0) membrane surface. In the longer time immersed experiments in bacteria suspensions for anti-biofouling performance (up to 10 d), both CS_Ag(+) and CS_Ag(0) showed much less biofouling than CS and they behaved almost equally good in their anti-biofouling behavior initially (24 h), but the CS_Ag(0) membrane surface gradually exhibited more stable and eventually better anti-biofouling performance than the CS_Ag(+) membrane surface afterwards. The results in this study demonstrated that the immobilization of silver onto membrane surfaces can be an effective method to improve a membrane's anti-biofouling property. The study was also the first of its kind that evaluated the relative anti-biofouling performance by immobilized silver in ionic and reduced states on a membrane surface. (C) 2010 Elsevier B.V. All rights reserved.