The sealing of tunnel walls involves the use of membranes which, depending on the situation, are constantly exposed to the leaching action of groundwater, or of water emanating from the surrounding rock. In terms of ecological product compatibility, the question arises as to whether pollutants may be released in the process. Various geomembranes are examined in more detail and subjected to an evaluation system. It emerges that in particular geomembranes with especially worrying components such as phthalates should generally be kept away from the environment for precautionary reasons, even though some phthalate plasticisers have not yet been evaluated by the EU. Phthalates are characterised by the fact that they are released from the membranes in considerable quantities.