In several countries low permeability crystalline rocks (e.g. granites) are under consideration as potential hosts for radioactive waste repositories. In such formations groundwater flow occurs predominantly in specific fractures rather than being a general porous flow through the entire rock matrix. By considering fractures to be simple parallel plates various authors have demonstrated the potential importance of diffusion into dead-end pores and the rock matrix itself (‘matrix diffusion’) as a mechanism for the retention of migrating radionuclides. Complementing these theoretical studies, several insitu migration experiments are planned in single fissures in crysalline rocks in Sweden, the U.K. and the U.S.A. The ‘parallel plate’ approximation to a single fissure is, however, acknowledged to be a gross simplification of any real case where “flowing” fractures are expected to be either filled or coated with secondary minerals, formed by hydrothermal alteration of fracture surfaces In the evaluation of net radio-nuclide retardation, therefore, the effect of sorption onto such secondary minerals must be carefully considered.