Geological clay formations are often considered as a host rock for a future waste repository. Many studies concerning sorption or desorption experiments of radionuclides or other toxic metals onto/from geological samples like clay are performed with batch techniques where only small amounts of the homogenised clay samples are in contact with the appropriate metals diluted in high volumes of aqueous solutions. This can lead to a high leaching of metal ions originating from the clay. The leached elements can considerably increase the background concentration of many isotopes, and they may also influence the results of the sorption/desorption experiments due to their competition with the used metal ions. This study focuses on this problem by analysing element leaching of clay during the batch procedure. After characterisation of the used Opalinus clay (OPA), element leaching was performed by extraction of homogenised clay samples from a bore hole with two different leaching solutions, Milli-Q water and, in contrast, synthetic porewater (PW) with a high ionic strength. After centrifugation, the leached elements in the supernatant were analysed by inductively coupled plasma-mass spectrometry (ICP-MS). Depending on the used extraction solution different elemental compositions in the leachate can be determined. The leaching of OPA with Milli-Q water leads to a dissolution of mainly anionic element species, whereas the use of PW leads to a release of mainly cationic metal ions. In particular, metals like Al, Fe, V, Cr, Co, Ni, Sr, Ba and U (beside the present PW ions) can be found in the leachates.