Selenium is both an essential nutriment and a chemio-toxic trace element for humans, animals and plants, the safety margin between beneficial and toxic doses being narrow. In addition, a radioactive isotope, 79Se (long-lived fission product, â emitter), is recovered in the High Level Nuclear Wastes (HLNW). Therefore, understanding selenium behaviour in soils is of major concern for both eco-toxicologists and radio-toxicologists. Biogeochemical transformations of selenium include sorptions/desorptions on solids, and abiotic and biotic reductions/oxidations that can favour its volatilization. Among the factors controlling selenium mobility, the redox status of the soil, mainly governed by the soil geochemical features and microbial activities, appears as a major one. This study aimed at understanding the effect of the soil redox status on the selenium mobility and at identifying the nature of the involved processes. More precisely, our objectives were to identify the main processes involved in the selenium retention in aerobic and anaerobic conditions, and to distinguish between abiotic and biotic processes The soil was a sandy soil coming from the Rhone borders (France), sieved at 2 mm for the experiments. Batch incubations were performed under laboratory controlled conditions, either on soil alone (control) or on amended soil with/without a preliminary sterilisation. For the amended incubations, the soil was supplied with a solution containing a mixture of electron acceptors (NO3-; SO42 -) and donors (glucose, cellulose) to stimulate microbial activities. Additional incubations were performed after supplying either a bactericide or a fungicide in order to distinguish between bacteria and fungi effects on selenium transformations. Radiolabelled selenite (Se IV + 75Se IV) at 10-6 mol/L was supplied either at the beginning of the incubation or later. Measurements were performed to (i) identify the distribution and speciation of selenium in the soil, (ii) assess the main microbial activities and (iii) characterize geochemical changes. The distribution and speciation of selenium were precisely determined by mass recoveries of 75Se in the aqueous, gaseous and solid phases. At the end of experiments, sequential extractions were performed in order to identify and quantify the dominant selenium fractions in the solid phase. Measurements aiming at giving information on the reduction/oxidation level of the soil included EH and dissolved O2. Other analyses were performed to characterize the microbial compartment of the soil (total microbial biomass, enumeration of total bacteria, quantification of the fungal biomass and analyses of the microbial community's structures). Analytical monitoring, reflecting the microbial activities, was performed in gas (O2, CO2, CH4, N2, N2O, H2), and in aqueous (carbohydrates, organic acid, anions and cations) phases. Experiments are still in progress. First results on the raw soil show a stabilisation of retained selenium with time. The comparison between sterile and non-sterile soils shows that the selenium retention results from interacting geochemical and biological processes. The distribution and speciation of selenium are affected by the soil redox status and microbial activities. The resultsare presented and the consequences of various identified processes on selenium fate in soils are discussed. Key words: soil redox potential - selenium - microbial activities - soil geochemistry