The process, thermodynamical isotherm and mechanism of Cr(III) uptake by Spirulina platensis were investigated in this paper. At the beginning of Cr uptake, Cr is physically adsorbed to the surface of algal cell by electrostatic attraction. Finally, chemical complexation through ion-exchange with K(+), Mn(2+), Ca(2+), Mg(2+), Na(+), Fe(3+), Zn(2+), H(+), etc. ions by reversible or irreversible strategies plays the main role in Cr uptake. Most of the absorbed Cr are steadily bound to proteins, polysaccharicles, lipids, etc. biological ligands, whereas only a very small part of Cr are loosely adsorbed on the algal surface. Langmuir isotherm model seems to be suitable for describing Cr uptake than Freundlich isotherm model. pH is the most important factor influencing Cr uptake as well as anions, cell concentration, Cr concentration, temperature, light. Scatchard analysis and isotherm analysis indicate multi types binding sites for Cr and multi interactions between Cr and alga. Bioaccumulation strategy is suggested to be involved in Cr uptake besides biosorption e.g. adsorption and chemical complexation. S. platensis is of efficiently capable in the uptake of Cr(III), consequently high-value Spirulina can be produced. (c) 2005 Elsevier Ltd. All rights reserved.