Clay barriers are routinely used in nuclear waste repositories to isolate radionuclides from the biosphere. Such barriers have to be carefully designed to remain effective for at least 300 years over potentially changing environmental and geologic conditions. There are two types of water in the clay interlayer water and inter-particle water. This study will estimate the diffusion of these different types of water. The diffusivity of the inter-particle water will be measured after collapsing the interlayer using a method developed in earlier work . It is envisaged that the cations do play a dominant role in binding water to the interlayer and effect of the cation on the interlayer water will be estimated as a function of the cation concentration in the interlayer. From these measurements it should be possible to make better estimates of the ability of clays to limit solute movement .