In porous media, the extent of dissolved organic matter (DOM)-facilitated contaminant transport depends on the concentration, conformation and the size of the dissolved organic species. Yet, these parameters are highly sensitive to the ionic strength (IS) and the ionic composition of the solution. Boom Clay (BC) which is considered in Belgiumas a potential host rock for nuclear waste disposal contains polydisperse DOM that might associate with radionuclide and increase their mobility. To getmore insight into the effect of IS on DOMstructure and into its impact on the solid/solution partitioning of OM in BC is essential for safety assessment. In a first set, we investigated the influence of NaCl and CaCl2 content on the concentration, the MW distribution and UV spectral parameters of DOM collected from BC. With an increase in IS two main mechanisms were identified: a compaction and/or dissociation of the DOM molecules and an aggregation.We showed that the sensitivity of the DOMspecies to these two mechanisms was size/MWdependent and that the presence of Ca2+ promotes the aggregation. The largest species are more prone to aggregation which at the extreme leads to their transfer to particulate OM. On the contrary, small DOMspecies hardly aggregate but compact or dissociate with an increase of IS. These observations were confirmed in the second experimental set in which we followed the release of DOM from BC rock in various electrolytes. The increase of IS and multivalent cations content reduces the amount, the degree of aromaticity and the MW of DOM released from BC which limit the extent of DOM-facilitated contaminant transport in BC.