BackgroundDiffusion in porous - and/or in confined media plays an important role in many different materials, like in membranes for fuel cells, in hydrogen storage materials, in micro- and nanoporous filters, in biological systems, etc.Water diffusion in clays is important in the context of radioactive waste repositories, because the pore water is the most likely transport medium for radioactive particles through the clay barrier into the environment.Clays are build up from disc shaped particles, and may contain water either in the larger pores between these particles and/or in a two dimensional space between crystalline sheets of these particles, in the so called interlayer. In the water, near to internal and external surfaces charge compensating cations may also be located.AimThe aim of this project is to describe the process of water diffusion in swelling clays.Main focuses are to understand the coupling between cation and water dynamics, to describe the anisotropy in thediffusion, and finally to investigate the effect of different parameters, like bulk dry density and particle size.Our approachIn porous materials the determining process for diffusion depends on the lengh-and timescale. We measure the water diffusion at the atomic level by quasielasticneutron scattering. These data will be evalutated according to realistic models,suggested by computer simulation. Furthermore, water diffusion is measured in nearly identical samples by tracer through-diffusion method over distances of centimeters. We could show in a previous project, that from such a datasetgeometrical and chemical effects can be separated quantitatively.Newly, also cation diffusion should be measured by tracer through-diffusion.SignificanceUnderstanding the process of water diffusion in clays is beneficial for the safety of radioactive waste repositories, and thus the protection of the environment. Furthermore,the quantitative separation of geometrical and chemical effects on the water diffusion can be transferred to other porous systems, being interesting for other reasons.