The bioavailability and toxicity of dissolved metals are closely linked to the metals' chemical speciation in solution. Normally the complexation of a metal by a ligand would be expected to decrease its bioavailability. The aqueous speciation of uranium (U) undergoes tremendous changes in the presence of ligands commonly found in natural waters (carbonate, phosphate, hydroxide, and natural organic matter). In the present project, links between speciation, medium composition, and bioavailability of uranium toward Chlamydomonas reinhardtii, a unicellular green alga, were investigated. Short-term metal uptake rates were determined in simple inorganic media at constant low pH (5.0) and hardness with particular emphasis on the differentiation between adsorbed and intracellular metal. While intracellular uptake was fairly linear over 1 h, partly reversible adsorption reached steady-state within minutes. Both adsorption and absorption were saturable processes (with a half-saturation constant Km of 0.51 μM). Addition of phosphate, citrate, or ethylenediaminetetraacetic acid (EDTA) as ligands decreased uranium bioavailability. No evidence indicating the transport of intact uranyl complexes was found (i.e., facilitated diffusion of metal bound to an assimilable ligand such as uranium-phosphate complexes). Within these experimental conditions, uranium uptake was correlated with the free uranyl ion concentration as predicted by the free-ion activity model (FIAM) and biotic ligand model (BLM).