The overall objective of CatClay is to provide a scientifically well grounded answer to the following question: Can the migration of actinides and other strongly sorbing radionuclides in clayrock be predicted by coupling models of (i) their sorption equilibria on representative clay minerals and (ii) the diffusion-driven mass transport of radionuclide sorbed and dissolved species in compacted masses of these clay minerals? The answer is of prime importance for Safety Cases for clayrock formation-based radioactive waste disposal concepts. The project takes as its starting point experimental observations showing that certain cations known to form highly stable surface complexes with sites on mineral surfaces, migrated more deeply into clayrock than expected. This suggests that current models may not be correct for these cations. CatClay calls upon a number of innovative approaches. It provides i) high resolution experimental methods leading to high quality, low analytical uncertainty data sets, ii) innovative conceptual, and corresponding numerical, models in order to represent the coupled diffusion-sorption behavior of surface complexing radionuclide. CatClay is structured along 3 RTD workpackages, combining modeling and experimental studies from the simpler, analogous system (clay) to real materials (clayrocks). The sorption-diffusion behavior of 3 cations will be studied selected according to their known differences in sorption reactions with clay mineral surfaces. CatClay combines the forces of 5 research institutions having proven expertise in experimentally studying and modelling the above described processes; the developer of a widely used coupled transport code; one waste management organisation for the application to safety case. The transfer of knowledge to a broader community, scientists and end-users, is also done through publication in peer reviewed journals and publically-available reports.