The affinity of chitosan for several metal anions (molybdate, vanadate, arsenate, chloro-platinate and chloropalladate) was investigated with the objective of identifying the controlling parameters (related to intrinsic polymer properties, diffusion properties and the properties of the metal in solution). Better understanding of sorption mechanisms and controlling steps may help not only in optimizing the use of these biosorbents but also in designing new chitosan-based sorbents. Some examples of physical modification (gel bead conditioning) and chemical modification (grafting of polyethyleneimine, sulfur compounds, impregnation with metal ions) are given to illustrate the versatility of the polymer and its promising performances for the recovery of toxic and/or valuable metal anions. (C) 2004 Elsevier B.V. All rights reserved.