Diffusion into the rock matrix is potentially an important retardation mechanism for nuclides leached from an underground radioactive waste repository in a fractured hard rock. A technique for measuring the intrinsic diffusion coefficient and rock capacity factor is briefly described. Simple solutions to migration model equations are used, together with diffusion results and typical hydrogeological parameters, to estimate the impact of matrix diffusion on radionuclide migration. It is shown that retardation factors in excess of 100 and reductions in the peak concentration by 3–4 orders of magnitude are possible for non-sorbed ions, which would otherwise be carried by the flow and not retarded at all.