When assessing a rock formation for its suitability as a potential host rock for the disposal of radioactive waste, knowledge about its pore water chemical composition is essential. When the HADES Underground Research Laboratory became operational in the early 1980s, it offered the possibility of extracting pore water from Boom Clay. At the time, however, there was almost no experience of sampling pore water from deep clay formations. The low hydraulic conductivity of clays makes it difficult to extract pore water and the sampling process itself can induce changes in the observed chemical characteristics due to oxidation and re-equilibration with the ambient air. In the past decades, significant progress has been made in the techniques and protocols to sample and monitor pore water. The use of nitrogen instead of compressed air to drill boreholes and installing piezometers limited the disturbances induced by oxidation of the clay. Furthermore, an advanced system was developed to simultaneously sample pore water and dissolved gasses and measure some key geochemical parameters such as pH, pCO2 and Eh at in situ conditions. This has resulted in a more reliable characterization of the Boom Clay pore water and a better understanding of perturbing processes such as oxidation.