It has been previously shown that selenium was poorly mobile in soil due to its strong interactions with minerals (e.g. clays, iron oxides, etc.); organic matter and micro-organisms. The objective of this study was to determine the impact of soil organic matter degradation on selenium immobilisation and/or redistribution among soil components. We focus here on the degradation of particulate organic matter (fragments of leafs, roots… bigger than 50 µm), one of the fastest decomposable organic fraction (few years). Size-density fractionation was largely used in order to evaluate the Soil Organic Matter (SOM) quality and its biodegradation within carbon turnover studies. This technique was originally used to easily assess both Particulate Organic Matter (POM) degradation and selenium distribution within the isolated fractions. The protocol used (based on Balesdent et al. (1991)) consisted in dispersion and wet sieving steps, followed by a density separation step. All these steps were conducted in water solution in order to limit selenium desorption and/or redistribution from/within the soil components. Finally, four different fractions were separated: organic fractions bigger than 50 µm (defined as the particulate organic matter, POM_>50µm); mineral fractions bigger than 50 µm; mixed organic and mineral fraction smaller than 50 µm; and an aqueous fraction. Three different soils with contrasted organic matter quality and content but with a similar mineralogical composition were incubated at a constant temperature and moisture optimised to accelerate the carbon turnover. Size-density fractionation was performed at different key times of incubation, corresponding to different degradation-states of the SOM. Selenium and organic carbon concentrations were measured for each fraction separated. The first results, obtained after 7 months of incubation, showed an evolution of the solid partition of selenium with an increase of selenium associated with POM_>50µm. In the soil with the highest organic matter content, 12 % of the total sorbed selenium was associated with the POM_>50µm at the beginning of incubation whereas 18 % of selenium was linked to this fraction after 7 months of incubation. As the mass of POM_>50µm did not changed during this period, results suggested an increase of POM>50µm-selenium affinity. Complementary analyses were performed in order to identify which compounds of POM_>50µm- were involved in the evolution of the “POM_>50µm-selenium” association. They consisted in Transmission Electronic Microscopy, titration of functional groups, and sorption experiments on the different isolated fractions. The latest allowed to further investigate the selenium reactivity with the different POM_>50µm fractions. Balesdent, J., Pétraud, J.P. and Feller, C. (1991). Effets des ultrasons sur la distribution granulométrique des matičres organiques des sols. Science du Sol, 29 (2): 95-106. Key words: size-density fractionation; particulate organic matter; selenium.