Percolation tests (i.e. routine laboratory migration experiments) have been performed to evaluate the diffusion behaviour of a number of long-lived radionuclides in Boom Clay, a candidate geological host formation for high level radioactive waste (HLW) in Belgium. Among the many potentially hazardous radionuclides under investigation are 79Se, 93Zr, 237Np, 241Am and 243Am. Actinide migration experiments have been carried out with 237Np and 241Am, while for the studies with Se and Zr the radioisotopes 75Se and 95Zr were used. Their transport patterns in the Boom Clay were examined using a combination of a NaI(Tl) detector and High-Purity Ge detectors either on the dissolved matrix (237Np, 241Am) or directly on the clay (75Se, 95Zr).