In situ migration experiments have been performed in the HADES Underground Research Facility (URF) in the Boom Clay below SCK•CEN at Mol (Belgium). Small-scale experiments were carried out with strongly retarded tracers e.g. strontium, caesium. The transport of these tracers can be described by the diffusion retardation formalism. The transport parameters derived from the in situ experiments can be compared with the laboratory-derived values. For both tracers, the apparent diffusion coefficients measured in the in situ experiments agree well with the laboratory-derived values. In the large-scale experiments (of the order of metres) performed in the URF, non-retarded or slightly retarded tracers (HTO, iodide and H14CO3 _) were used. The migration behaviour of these tracers was predicted based on models applied in performance assessment calculations using migration parameter values measured in laboratory experiments. These blind predictions of large-scale experiments agree well in general with the experimental measurements. Fitting the experimental in situ data leads to apparent diffusion coefficients close to those determined by the laboratory experiments. The iodide and H14CO3 _ data were fitted with a simple analytical expression, and the HTO data were additionally fitted numerically with COMSOL multiphysics, leading to about the same optimal values.