Selenium (Se), a sulfur analogue, is present in soils in trace amounts. It is both an essential nutrient for many living organisms and a potentially toxic element. Areas of low and high contents have been identified worldwide as well as areas polluted by human activity. Furthermore, one of its isotopes, ⁷⁹Se, is an important component of radioactive waste and its fate in the environment must be assessed. Se is mainly present as SeO₃^2- in the soil solution in the usual range of pH and Eh. This ion is readily taken up by plants and so can easily enter food chains. It is weakly adsorbed onto soil constituents but can be reduced by microorganisms under aerobic and anaerobic conditions to immobile elemental selenium Se° and to methylated volatile compounds, that are less toxic and water-soluble. Microorganisms also contribute to the formation of poorly identified complex organic compounds. Some abiotic transformations to less mobile forms have been described under controlled conditions. The extent to which the immobilisation of Se is reached in soils under variable conditions of aeration, moisture and microbial activity is poorly understood and so it remains difficult to give accurate predictions of Se mobility, necessary for risk assessment. The aim of this study was to follow the vertical movement and immobilization of Se in soil columns and to relate these to water movement and to the redox state and the corresponding degree of anoxia as a function of time and depth. Both native soil Se and Se added as a soluble SeO₃^2- salt (2 mg Se kg^-1 soil) were studied. The effects of tomato plant growth and straw addition were investigated. Water potential and the composition of the soil atmosphere were monitored throughout a three-month period. At the end of this period, the soil columns were dismantled. Half of each column was resin impregnated and aggregate size distribution calculated from image analysis. The other half was used to measure moisture content and Se fractionation with a simple parallel chemical extraction scheme - (i) water soluble, (ii) exchangeable, (iii) pyrophosphate extractable and (iv) total Se. The anoxic fraction was calculated as a function of time and depth from the aggregate size distribution, water content and respiration. One set of columns was enclosed throughout the two-month period in a bell jar with a uniform flow through of air with constant composition. The outgoing air was bubbled through a solution of NaOH-H₂O₂ to trap any volatile Se. Losses by volatilisation were measurable but small (about 0.12% of soil Se accumulated over the 3-month period). Vertical movement upwards of total Se was small and consistent with water flux and the proportion of Se in solution. Little Se was extracted by water and the content decreased with depth as redox potential fell. The ratio between water soluble and exchangeable Se was similar for all treatments and changed little with depth, except near the water saturated zone where it increased in line with the drop in water soluble Se. The presence of a plant or straw did not markedly influence the chemical associations of Se. The anoxic fraction varied with depth but changed little with time. This fraction will be compared with the chemical fractionation of Se to better understand the immobilization processes.