The initial water saturation in North Sea chalk reservoirs is extremely low, often in the range of 5-7%. Outcrop material is often used to perform parametric studies on wettability and secondary oil recovery by water injection. To relate the experiments to a certain reservoir, it is desirable to mimic the reservoir conditions as closely as possible. This is done using outcrop cores. The cores are usually drilled in the same direction from the same block, dried at 120 degrees C, evacuated, and saturated with the actual brine. Homogeneous water saturation can be obtained by draining the initial brine to a saturation of less than 10% by using water-saturated nitrogen gas and a porous plate. During this process, the water saturation is decreased from 100% to less than 10%. Potential determining ions like Mg2+, Ca2+, CO32-, and SO42-present in the brine or as salt initially present in the core will adsorb onto the chalk surface. Thus, during the drainage process, the potential determining ions are increased in concentration inside the core, and the apparent concentration in the residual water becomes different from the initial brine. The wetting condition obtained during an aging period at 90 degrees C is very sensitive to the concentration Of SO42- in the initial brine. Outcrop chalk cores that were preflushed with 4 pore volumes of distilled water to remove SO42- appeared to be much less water-wet compared to cores not preflushed in the same way. The amount Of SO42- present before and after flooding with distilled water was quantified. A new standard procedure to prepare outcrop chalk cores for wettability studies is suggested: (1) The cores must be flooded with distilled water or formation brine without SO42- to remove SO42- that may be present initially. (2) The initial saturation brine must not contain SO42-.