For the disposal of High Level Waste (HLW) in a deep geological formation as Boom Clay, safety assessment studies have shown that long lived 79Se is one of the more critical fission products. Therefore, the knowledge of its migration properties (diffusion, retention) through the geological barrier (Boom Clay) is of paramount importance. The migration behaviour of selenium strongly depends on its speciation. Under reducing conditions, selenide would be the dominant species and selenium migration would mainly be controlled by the low solubility of Se(-II)-bearing minerals. However Se species are often found in redox disequilibrium and more oxidized species might also coexist. Therefore, the study of selenate migration requires attention, as it might be the most mobile selenium species in the host rock. Electromigration experiments performed with a 75Se-labeled selenate in Boom Clay indicate a high mobility for this species. The apparent diffusion coefficient (Dapp) of selenate in Boom Clay is estimated from electromigration experiments performed under different electric fields. Using two independent approaches, the value of Dapp for selenate is shown to fall in the range 1.7 × 10-11 to 6.2 × 10-11 m² s-1. Moreover, no reduction of selenate in Boom Clay was observed.