The uptake of uranium(VI) by natural pyrite, FeS2, was studied under conditions relevant for geological disposal of radioactive waste (anoxic atmosphere, ~0.014 mol • L-1 NaHCO3 electrolyte) with special emphasis on the role of dissolved organic matter. Solution analysis of batch experiments with different initial concentrations of uranium(VI) (10-8-10-4 mol • L-1) was combined with X-ray absorption spectroscopy on the solid phase to elucidate the speciation of uranium in these systems and to gain insight into the major reaction mechanisms between uranium and pyrite. The results showed that, under the conditions of the experiments, uranium(VI) was at least partly reduced to a UO2(s)-like precipitate, although the predominant valence state of uranium in solution was likely uranium(VI). All observations indicate that the uranium solid-liquid distribution is governed by both reduction and adsorption processes. No significantamountsofuraniumcolloids(eitherintrinsicUO2 colloids or complexes with natural organic matter) were found in any of the samples. The presence of dissolved organic matter did, however, increase the final uranium solution concentration and decrease the fraction of uranium(IV) found in the solid phase.