[en] In Belgium, the Boom Clay is considered as a reference formation for methodological studies for deep geological disposal of high level radioactive waste and spent fuel. The Boom Clay has many favourable physical and geochemical characteristics, like diffusion-dominated transport (very low permeability), and pronounced capability for an efficient retardation (by sorption processes) of mobile radionuclides (slightly alkaline pH and reducing conditions). A focussed research programme was started on radionuclide sorption on Boom Clay with a two-fold goal: 1) to acquire and select sorption parameters that serve as an input for safety assessment calculations 2) to scientifically underpin the selected data by demonstrating a comprehensive understanding of the radionuclide uptake processes in the host formation. This latter goal necessitates the ability to sufficiently describe the most relevant radionuclide uptake processes by means of quasi-mechanistic chemical formulations. In Boom Clay pore water, the most important reactions for lanthanide and actinide ions influencing their speciation and mobility, are hydrolysis and complexation with naturally occurring ligands such as inorganic carbonate and dissolved organic matter. Interaction with dissolved OM generally leads to a decreased sorption to the mineral surface, thus increasing the mobility of the radionuclide. A good description of these interactions is therefore needed for making trustworthy predictions. The objectives of this study were to obtain high quality sorption datasets for representative trivalent (Eu) and tetravalent (Th) RN (radionuclides) on Boom Clay while scoping the influence of organic matter on the sorption behaviour and to describe RN uptake on Boom Clay in presence of OM by a (quasi)-mechanistic model which enables predictions to be made