Several experiments were set up to study Se speciation and solubility in the reducing Boom clay environment, starting from oxidized Se species which were added in oversaturation with respect to the thermodynamic solubility of reduced Se solid phases. Upon introduction of SeO3(2-) to FeS(2)-containing samples, adsorption of SeO3(2-) occurred at the FeS2 surface, and led to a reduction and precipitation of a Se0 solid phase with a solubility of 3x10(-9) M (after 60 days). In the presence of humic substances, an association of Se with these humic substances was observed and the 3x10(-9) M solubility limit was not reached in the same time delay. Upon introduction of SeO3(2-) to Boom clay suspensions (equilibration up to 9 months), the initial adsorption of SeO3(2-) on the solid phase was increased with respect to systems containing only FeS2, due to the presence of (illite) clay minerals. This competing adsorption process, and the presence of humic substances, again decreased the kinetics of reduction with respect to FeS2 samples. Also, an association of Se with Boom clay humic substances was observed, and amounted up to approximately 10(-7) M in some samples after 9 months equilibration.