Pretreatment of biosorbents have been suggested to modify the surface characteristics which could improve biosorption process. Stable cesium biosorption was studied in continuous fixed-bed column by chemically modified biosorbents. Two kinds of brown algae (Sargassum glaucescens and Cystoseira indica) were treated with chemical agents including formaldehyde (FA). glutaraldehyde (GA), potassium hexacyanoferrate (HCF), FA and HCF, and CA and HCF. The highest biosorption capacity (BC) was obtained from C indica treated with FA (63.5 mg Cs/g biomass) and S. glaucescens treated with FA and HCF (62 mg; Cs/g biomass). To study the effect of the best treatments on the BC, the concentration of each treatment agent was decreased. With decreasing FA agent for C indica treatment, the BC dropped. Treatment of 1 g S. glaucescens biomass with 2.2 g FA and then 0.18 g HCF resulted in the highest BC (73.08 mg Cs/g dry biomass) which was 35.8 times higher than intact S. glaucescens. (c) 2008 Elsevier B.V. All rights reserved.